I have always wondered, what are the main differences in the same model engine on/offroad versions. As I understand it, the companies use a lot of the same components but change, carbs, timing on crankshaft/sleeve height. What are the most obvious differences between the two models? Is it possible to modify an off road engine so that it performs as fast as its onroad counterpart? Thanks for your time and patience.

It's possible, but not worth the effort. Sell the off-road engine to someone who can use it in their truck, and buy an engine suited to your vehicle.

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.

There are many things mentioned about port time/area.....the faster an engine turns then the more it will need..but you get nothing for free.