Had the same issue. Actually lost the nut on the opposite side. Fixed this issue by using a nylon locking nut and blue thread locker and tightened it up a little more than it should be.i had major vibrations, was going to change exhaust.... but put in rubber washers between all mounting bolts, helped some, then later on i noticed that the chain was making noise, pulled off sprocket cover and found motor mount bolt had come loose and was bouncing around just behind the sprocket. looked like the nut came off the RH side, my assumptions was that the guy who installed the exhaust might have thought is was part of the mounting for the exhaust and forgot to tighten back up after done.
Do you know what size bolt you used? I am having the same problem and can't find any dimensions listed anywhere to buy a replacementHad the same issue. Actually lost the nut on the opposite side. Fixed this issue by using a nylon locking nut and blue thread locker and tightened it up a little more than it should be.
I had this issue and thought it was the motor bolts being loose too, so I tightened them too much and they snapped -- Don't do what I did...
FunnyNext time why don't you do yourself a favor and pay more attention when you're dismantling something, so you can remember how to rebuild it properly, or better yet, pay a professional to do it for you... That way it's less likely to fall apart while you're riding it due to lack of properly torqued fasteners.