9mm blowback, using a rewelded Apex Galil receiver. I'm still reading up on the trust and Form 1 stuff.
edit: Jack Squat PPSh-41 magwell, PPSh-41 mags. The PPSh bits look more similar to the Magal ones than the Suomi bits. Several people have mentioned the PPsh mags handled 9mm with no trouble.
The most difficult/expensive part will be the folding stock. The Magal uses an uncommon variant of the Galil stock, and I might wind up having to fake that unless I can snag a deal on the right parts.