It depends the exhaust port. If it has a M.E.S. (2 small side exhaust ports) then a lot is possible and indeed no problem. Just copy the same timings of an onroad version and you are on the way. Change the carb to an 9mm onroad version and it must run like an onroad. I have modded a JP 8P offroad once with the specs of a JP R09, it did run the same.
If it has a normal 1 port exhaust system then still a fast engine is possible but not as fast as a real onroad.