Rubber o-rings, pistons, springs, pins, maybe assembly groups like a spare trigger pack or bolt carrier group.
Brownells sells a lot of these parts, but they’re not cheap. Things like trigger packs and bolt carrier groups will send you into a seizure when you see the price. If deciding to build another trigger pack, I’d fit an aftermarket one the way you like. Then piece the oem parts you’ve removed into its own assembly.