I assume that this is a standard straight cut rear.

What are the stock replacement options and how much modification is necessary to implement them?

http://pic15.picturetrail.com/VOL591/2748309/5504459/70484600.jpg

any stamed rec stockset will work mods will b vary vary small on the maks u usally have to trim the sides down alittle just dont trim much make shure its a really tight fit

BTW to b legal u have to add a buttload of us parts to change from a butthole stock (altho ive never actually had anyone count my us parts it still kind nice to know its legal)  

Keng's

I am staring off with a receiver like the one shown in the photo. I started by cutting off the rear trunnion flush. I then centered my hole off the 2 trunnion pins. after all togeather I will make a plate to close the hole. I was going to cut the receiver at a angle but did not. I had to go in and remove metal from the trunnion. Metal was removed from the top and from the rear to give me some room to put pins in. It will be a couple days before I can finish up but will try posting photos when done.

PS 7 US parts put in to gun before I started.