I don't really know if this has been addressed before, but I thought I'd ask. I recently scored a deal for a Norinco Mak 90 sporter and the guy that previously owned it had it converted to an underfolder style. I just took off the stock and with the GIANT gaping hole in the rear of the receiver....I was wondering if there is any way to fix it or would it be recommended to purchase a new receiver? Yeah, I'm kinda new to the AK scene but have worked on plenty of ARs, 1911s, bolt rifles & shotguns....the AK is way different than those so I figured my best bet was to inquire with the experts. THANKS!

Well there are some holes common to the underfolders and there are other holes common to other style builds.  A more precise description or picture would be necessary for most to answer your question.  

Ok finally managed to get some pics, but it's from my phone :( I really hope it works. As you can see I've done a little patch work with some quik steel, but I'm not pressing my luck on that stuff, I also took the old pin and packed quik steel in it so it just kind of fills the hole....but i'd rather have it look good instead of looking like something out of Fallout 3 I've also noticed that the 1.5 (or 1.6) mm receivers for chinese aks are a little hard to find but I did find a place.
http://i1131.photobucket.com/albums/m546/tacticaloperatr/IMAG0702.jpg http://i1131.photobucket.com/albums/m546/tacticaloperatr/IMAG0701.jpg

That was a factory underfolder from Norinco. To keep them coming in after the ban they just knocked the stocks off them and put the big ugly thumbhole stocks on them, covering up the hole.
If I were you I'd just pick up a factory Chinese underfolder stock assy and put it back on there. You'll have to do a bit of parts swapping do keep compliant with 922r but it should be able to be done.
PolyTech Parts had new factory Chinese underfolder stocks available, and the little pieces for the folder assy are available through other places.

You mentioned that you found a source for Chinese style 1.6mm receivers. Can you please tell me where? I've got a Chinese parts kit that I'd love to get put together but have been unable to locate a 1.6mm receiver for it.

ok thanks, again I'm new to the AK stuff, I just researched the 922(r) issue and I may be able to switch the receiver, I'm consulting a gunsmith now...but it's kinda hard since I had a wisdom tooth pulled yesterday thanks again, I'm gonna see if I can patch up the receiver a bit and make it work. also here is the best websites I have found.

http://www.cncwarrior.com/comersus/store/comersus_viewItem.asp?idProduct=203
http://hornscustomrifles.com/Receivers.php

If you decide to switch the receiver out, let me know. I'll buy that one from you and help offset the cost of the swap.
I have a Chinese parts kit in need of a receiver and would like to have the underfolder like you have.

sorry it took so long, been  dealing with the army reserves and an application process with a local pd. I decided to put the underfolder back on and am debating on keeping it and just buying another ak later or one of my friends wants to buy it... the only other ak's i've been looking at are the bulgarian and WASR models...but i've heard mixed results with the WASR.

The Bulgarian will be your best bet, the WASR is very hit and miss, you may get a good one you may not.
If you or any one else is going to buy a WASR be sure to look it over first, try shoot it if you can, there are a lot of bad ones out there.

I agree either build a Bulgy or buy one I haven't seen any WASR I haven't had to fix for a couple of my friends..... ( ok i know only two) but no one has asked me to help them fix their Bulgy...

Actually I just picked up a romanian draco pistol and boy does it shoot like a dream! Unbelievably accurate too so I'm thinking about waiting a bit and going the SBR route. I even have my inspiration.
http://i725.photobucket.com/albums/ww254/Gardner37/DSC_0021.jpg
Thanks so much everybody for all the help! It's always such a pleasure talking with the tactical community....I don't know why some people are afraid of it