I would probably get a G&P GB shell and use prommy internals.
The AR15 G&P hop unit is also decent too from memory, pair it with a Prommy 6.03 inner barrel an Prommy hop bucking and nub (purple).
The G&P motor was pretty decent too, as were the Systema's.
Tech, have had no real issues with John @Wolf Armouries, he's solid but he's busy.
I know Systema get's a lot of bad stick, but donkey years ago I built a SR16 with all Systema internals.
It had a Revolution gear box and motor, Systema hop unit and inner barrel.
It wasn't quite scratch build though (GB was pre built and was a meant to be drop in upgrade)
The thing ran flawlessly for a long time, accuracy was amazing and trigger response was crisp and snappy without a mosfet.
It's a shame that these parts are not as widely available as they were.