I want to do a kind of 'Khyber' look to my SLR107 with the triangle folder and laminate handguard but I have a problem. I'll be one part over compliance.
The fcg and furniture are US made, bringing the count to 10 foreign parts with a surplus mag. I'm thinking an IWD handguard and US grip but the only source for a good triangle stock with a 4.5mm pin is the Ruskies through KVar.

I want to keep the Bulgarian brake and use surplus mags, what's my best option?

922(r) is retarded

Buy 1 Tapco mag to have with you should it become necessary to prove your compliance. [Never happen in your lifetime] Remember, the gun w/o a mag inserted is compliant. At least that's how I calculate it.  GARY

A US piston?

Or this...easy "invisible" way to up your U.S. parts count.

You can use a US made magazine floor plate and follower.

This.  

US made mag parts as the other have indicated is easiest.  You can get US made floorplates and followers that work in on all the import mag bodies.    When I install original furniture I go with 1) FCG (3 parts), 2) US made floor plate (1), 3) US made brake (1), 4) US made pistol grip.   You can swap any of the above for a US made follower.

The Arsenal US made floorplates and followers work like the originals on the waffle mags. These floorplates also work on the bakelite mags.   Tapco followers have worked for me on everything I've tried and CSSPECS has US made floorplates for steel mags.

http://stores.csspecs.com/