I have ran the original CMB motor in my MRX3 2 years ago when it first came out. It was a torquey motor, lack top end, but has hell a lot of torque, good club racing motor for the $. Didn't like the carb, it was a Picco-esque carb, I put a Novarossi carb in it and it worked fine.
You can find info on CMB here.
http://www.reidrc.com/on-road.htm
Dom