This was my fix. It held the gearbox in place just fine but i definitely could have done better if the tolerances were right. The metal i used was this really soft sheetmetal that i scrapped off an old computer PSU but once i clamped them together it was a solid piece and reinforced to prevent further breakage. I lost the pieces of plastic mind you and i didn't have the tools to put it back together in the same place.
You'll probably need to find something else like polycarbonate and use a solvent to weld everything together. Weldon-16 is a godsend for airsoft since it will weld most ABS/PC plastic that is used to make these non-metal parts.