I had a friend that put a R motor in a mojave frame, worked ok, alittle short for that kind of power. I think he used a stock ATC pipe. I dont see why it wouldnt work, its not gonna be a great handling race bike or anything but would work on the trails given the 300EX's short wheel base and good handling manners.
"you always gotta have ur ambulance scene in every moto vid"..sean H-4
1988 Honda TRX250R ground up project
2008 Yamaha Raptor 700 SE- Stock
Laegers narrow CR500 link frame (just the frame for now)