Treetop is on the money.
I bought my upper from a board member, the L1A1 cocking handle I had but these can be picked up from various vendors or check the marketplace on FALFiles, usually around $15 to $20.
You need to modify the cocking handle by shortening it, you will need to remove approximately 1 1/2" to 1 3/4" so that when it is in the forward postion the tail end centers on the recess for the bolt catch. Smooth and square the end and round all corners and sharp edges to prevent them from gouging into the receiver grooves.
You will need to check the fit of the handle in the grooves and slim it down as needed, mine was already reduced in thickness to fit a metric FAL receiver but I dont think this is necessary for the AR upper, I did have to reduce the overall width on the actual rails outside to outside, it may also be necessary to reduce the width of the actual handle slide or body but probably not. Smooth and polish every contact surface and round off any sharp edges, lube it up and work it in the receiver to break it in.
The folding handle will need some modifications also. The inner (when closed) edge that fits into the notch in the receiver will need to be filed down to allow the handle to fold in parrallel with the receiver, again smooth and polish so it does not gouge, etc. You want it to be able to catch the rear of the notch and prevent rearword motion but it will be best to remove sufficient material so that it does not actually contact the recevier when closed if possible (some handles will not stop against themselves and will still go in to hit the receiver, install a small piece of rubber on the receiver to protect it), when the handle flies forward it folds automatically by inertia and can translate a fair impact to the receiver if you dont prepare for it as above, probably wont do any material damage but its worth taking into consideration.
The folding part of the handle will also be to long on the average L1A1 handle and will hit the rear edge of your free float tube, remove enough material from the end to prevent this and re-contour to match original profile, smooth and polish.
When you have made all these mods to the handle and it slides freely in the receiver you are ready to finish. polish it all up however you like with crocus cloth, wet/dry sand paper, buffing wheel, etc. and cold blue or finish with paint. Personally I recommend cold blue as paint or parkerization will alter the rail fit and cause friction and or wear to the reciever (especially from parkerization).
Now on to the carrier.
Install the carrier into the receiver without the handle for now. Pull it back so you can see the front edge, now take a sharp scribe (as fine as you have for better accuracy) and scribe the carrier along the top and bottom edges of the opening in the side of the reciever, get as tight to the edges of the slot as you can and scribe the lines about 1 1/4" back from the front edge, make sure they are visible.
Now push the bolt carrier into battery (I forgot to mention the you should have the barrel installed with the float tube at least hand tightened in place, preferably final torqued), install the cocking handle and slide all the way forward and fold the handle against the receiver. Now take your thin sharp scribe and insert it through the hole where the carrier lug goes, scribe around the perimeter against the carrier. When you remove the carrier you will see the lines scribed along the receiver slot and the circle from the handle lug hole, your carrier will need to have a slot cut to slightly wider than that width (not critical as long as it is at least a few thoudandths wider than the reciever slot, to wide wont hurt) and as far back as the back side of the circle towards the back end of the carrier (you have to have a few thousandths minimum space between the handle lug and this spot on the carrier so that when the bolt goes into battery it does not hammer the lug forward damaging it and or the receiver, not to much play though).
You can now take the stripped carrier to your local machine shop or machinist buddy and have the slot milled or you can use a dremel if you have the skill (not super difficult but you want it to be decent). The depth of the slot will need to be approximately a 10th of an inch, deep enough for good engagement of the lug but not all the way through the carrier (that would necessitate replacement of course and thats not cheap), so if you have any doubts about your dremeling capabilities take it to a mchinest.
If you dremel set it up in a padded vise, tape off everything to keep out garbage from the dremel (easy clean up) and place tape up close to your scripe lines (it will make it easier to see them and also will help prevent overrun and scarring of your carrier outside the lines).
Install a thin cutoff wheel (emory? the type that breaks very easily so have a good supply, if your carefull though you can do the job with one). Carefully start your cuts along the top and bottom, start out light and continue to deepen the cuts till it is close to final. Now start making additional cuts through the material between your outside cuts to approximatley the same depth, keep working at the material that is left to completely open the slot. Stop yor cuts just short of your rear mark as this will be finished later. Look at your slot carefully and dremel down any high spots to even it out as much as possible.
For this part you will need a dremel tip that has diamond bits impregnated into the sides and end, you could use a carbide end mill type cutter but those little shavings are like super fine needles and hurt like hell when you get them in your hands (ask me how I know). Carefully start to finish the end of the slot back to the line (cut so the line dissapears and that should give you the clearance you need), work the cut to the width of the slot and deepen the end where your cuttoff wheel left a ramp until its the same depth as the rest of the slot, work slow and use both hands keeping a death grip on the dremel for maximum control. Take it slow and easy and you will get nice results. After your done with the dremel find a good sharp triangular needle file, start working on the bottom of the groove till its flat and all the "mill marks" are gone, now true up the sides of the slot in the same manner. You can leave it as is or touch up with cold blue (recommended).
Now for the final fitting. Clean the carrier and reassemble, insert into the receiver and lock the bolt home. Install the charging handle and slide all the way forward. Insert the lug into the handle to see how it fits, if it protrudes out so the pin cant be intalled or it drags in the carrier slot you will need to file and polish the bottom edge down to fit (you want a little clearance so the carrier does not drag on it during cycling), check to see that you have some for and aft play in the handle, if the carrier is tight against the lug, file a little off the backside for a little relief. Touch up as needed with cold blue.
Now lube everything as normal (dont forget the charging handle slots) and try it out, pull it all the way back (the back end of the handle slide should NOT touch the lower buffer tube ring) and let it fly forward, the carrier and bolt should lock freely and the folding handle will close (if the spring is tight in the handle it might not self close till it breaks in a while but will eventually).
Thats it, sorry for the long diatribe, if some parts arent clear let me know and I will clarify. I wont post pics right now but will later of the finished product when I am able.