oakrodent has a great build using BRP parts. I also have done a tutorial on using newly made rear sheet metal, it is on the weaponeer site. your only real options are on the FCG and grip stick. There are 3 or 4 out there you could use. I used the BRP grip mods with the ar hammer, trigger, and sear. But, i am a big FAL fan and know that FCG quite well. I like the fact that the hammer spring in that set up is beefy already so it will reliably fire the harder surplus primers,plus the hammer shape is already close to being what it needs to be. Of course, the ar set up i used does also with the heavy spring. The main thing is to install the full auto bolt blocker and weld it in underneath the grip stick attachment assembly, which might mean that you need to take yous off then put it back on with welds to be 100% legal. That would be considered not readily restoreable as compared to drilling it out and putting a piece of rod through it and the receiver and welding. That method could be removed easy and might not be totaly legal. I would say read as much as you can here, and ask questions. This is a very friendly site and very informative. The prints are avaliable on BRP to view for free as well as the semi auto owners manual with the ATF letter. Hope this helps.
Measure, think about it, measure again, think about it some more, measure again, then cut......."Oh S***!!!!"