Nice score! You got a screaming deal. Be sure of your states legal length and SBR laws before undertaking any permanent mods. The pistol grip screw from the t/hole is not usually long enough. Find a USA grip and screw, the Chicom grips bring a premium. [If you can find them]
AS for a folding stock, I do not believe just any will fit as the chinese did not use any inner reinforcement. The holes in the receiver are all that is needed. Here is one that was listed on GunBroker http://www.gunbroker.com/Auction/ViewItem.asp?Item=41259170 By the way, the rifle shown in the cantraband thread in the chinese section was mine.
Good luck and enjoy the process 'cause it is going to be a great underfolder!
Oh as for the filler piece where the under folder goes, get a good size punch and smack the head of the rivet. The 2 eare will bend easily over and you push it out the other side.