This is why you get yourself an Elite-RC motor mount.
You only have to secure the motor to the (sliding part) mount once, with either M3 or M4 screws, whichever your motor accommodates. Just make damn sure you use blue thread-lock, and that your screws are as long as your motor's holes will allow.
And an important tip for using that mount: only use short pinions, such as those from Robinson Racing or Hot Bodies. Longer ones, like the ones included in the Associated e-Conversion, will not make full contact with the spur gear.