I took my MP5 gearbox apart and replaced some broken parts and put it back together. But the motor won't turn the gearbox over it runs but it want move when it is in the gearbox.

What did you replace? Did you re-shim? Did you wire your motor in reverse? Check you motor height. Check your fuse and wires for shorts. I need more info than "it doesn't work" in order to help. What about your AEG that doesn't work?