I would recommend to get a new motor. The stock motor isn't all that great
And the question about bolt catch....kinda hard to explain but it consists of 5 parts. Three of the parts are on the gearbox; little spring, the locking piece and the screwthat holds it together. You have to bend the spring like an L shape in that nook on the gb. Then screw it together. That's the best I can describe it lol. Then the bolt button on the receiver goes in and make sure the arm will go through the slot of that locking piece screwed onto the gearbox. Lastly just put that bitch pin back in.