Originally Posted by Roelof

Timings.....

Because offroad driving is more about driveabillity and torque it is better to have a low exhaust timing. That will keep the top RPM low and the bottom torque high and with that a wide torque band.

To get the best out of onroad RPM is needed, this means a high and wide exhaust port (M.E.S.) and also larger timings on the intake to get enough fuel inside which is needed.
Sadly with an onroad setup the bottom torque is less so a high engaging clutch is needed for a fast acceleration.
The high speeds of an onroad and the low placed bodies is giving a lot of airflow giving a lot of cooling, that is why onroad engines can work with smaller cooling heads.

But yes, an onroad engine can be used in an offroad and otherwise. The type of exhaust (mainly the length of the manifold) is also giving some changes to the engine, a longer exhaust is giving more torque and a shorter exhaust more RPM. In some ocasions is is also done, an offroad engine in an onroad car on a short track is a good choise.