Nuts I think are zinc coated (yellow-ish plating) and brass would work as well. They are tiny, maybe 4-5 mm across (from memory)
Screws in the pistons are phillips pan heads with the faint "x" bisecting the socket. No screws anywhere else on the greeblie side. It looks like they may have cast one of the chopped motor coil pieces with a phillips screw plugging the hole, though I'm not so sure. They are both glued on like the motor coil plate. Also I hope you got an old kit! The kits from the 90s onwards don't have the motor coil plate in them
Remember with the flash hider, all black, all silver, and then dust black over it. Get a fresh sharpie too!
You are on a freaking roll with these blasters! they look really great