^ Also consider the EG1000, while not as fast as the M120 by G&P, the EG1000 is a hybrid motor, so you will also see a larger reduction in trigger response with the EG1000, over the G&P M120, regardless, I have seen the M120 in action, and it is a very high performing motor, any choice would be good.
Have you also thought of a 12C lipo? Firefox sells them in a stick format and in a crane style. I have one in my AEG, and it is excellent, best of all, these LiPos have a lower discharge rate than most lipos, thus your gun can take these without modification. You would need a balance charger though.