In order:
1) No, no one is making a forward assist A1 type upper any more. Bushmaster advertises one, but it's actually a cuurent Canadian version, and has the ejection deflector bump on it. You've got two chances to get a real A1 type upper. Colt did put out some Sporters, right at the end of their run, w/forward assists. That wouid be easiest, since they have the big hole hinge. I found one at a gun show, where apparently someone had converted a sporter to an A2 upper. Have also seen a couple advertised on site here.
Other possibility is a GI surp upper. Found a total A1 upper (also at a show), surped out from A1 rifles being upgraded by the military to A2 specs. Again, you'll find stuff like this at shows, or listed on the for sale section of sites like this. Patience is the key[:D].
2}Yes, you can use an A2 style upper, but of course that would defeat your intended purpose of building an A1 clone. You'll need an adapter type hinge pin (also for a GI surp A1 upper).
3) There is no difference between SP1 and any other uppers, except that later ones have more metal cut away. Absolutely use your existing pieces. Being closer in weight to the original M16 pieces than what's out now, they'll put less stress and wear on your rifle.
4) Abolutely, your existing upper has value. Check the for sale sections here for awhile to get some idea of it's value, put it up for sale and you'll probably get in sold in 24 hours[:D]
I, for one, support your intentions. Over the past years, I've put together clones of all the major variations of the M16 used over the years. Did a XM177E2 clone on an SP1 carbine 20 years ago, then did an M16A2 lookalike 5 years ago. More recently, an M4gery, and during the past year an M16A1 clone.
Had a ball[:D]