Looking to replace the stock gearbox and revive my Classic Army M15A4 CQB. Still in great shape externally, but looking to replace the entire gearbox. After a few weeks of troubleshooting the stock gearbox and replacing every internal part I am still losing compression somewhere so its time for an upgrade. Does anyone know what newer gearboxes are compatible? I have tried a VFC gearbox and it would not fit the body without modding (which I don't want to do). Any info would be great, thanks in advance.