Probably not the best suggestion but I have always been partial to the RZ350 2 smoke.
Pros, the motor can be built to rip and is pretty strong right out of the box, lower center of gravity I'd think.
Cons, fuel mixing, mileage, more wrenching...
Okay so I'm not Super Man. I'm just the jack of many trades, master of some yet to find and as the world turns that would only be for a matter of time.
Okay, screw it, stop the world I'm getting off.