Or wrap some tape around the motor tower? (Or brass tube it) which you will want to do anyway?
The alignment does not really matter at this stage because the grip will align it afterwards, you just need to worry about the height for the shimming (as in how deep the motor is going into the gearbox when it is more or less centered in the hole)
Once you are centered in the hole, the bottom of the motor will sit at the bottom of the grip, and these two points should automatically align the motor correctly