when you tighten the motor mount screws, make sure there is a very small gap between the gear teeth, just enough slack to keep them from grinding each other..
as for ruining anything serious, its all replaceable so theres no permanent damage... with that kind of cutting and make-do work, theres no guarantee because it wasnt made that way. but if something breaks, there are parts everywhere for that mugen car..
just bashing around, at the very least make sure your motor doesnt run hot and youll be fine. if at all, run it rich where you can see a good trail of smoke upon take-off, at least you know its ok that way. change the filter with a new freshly oiled one when the other looks dirty, and make sure there are no ways for dirt to get into that engine, bc thats a nightmare waiting to happen!
the motor may or may not hold like it is, id lean more to not gonna hold, but you never know, its just something i wouldnt take a chance with if i were RACING, but since your just bashing around, if it breaks, not such a big deal, lol! but whatever happens, have fun man!