The size of the bearing has nothing to do with the kind of engine, it is just an evolution and at the same time a saving of their own market. The 14 x 25.4 bearing is used with most brands and if you have the choice between a Picco bearing for half the price as a Novarossi then the choice is simple made.
The difference between onroad and offroad is indeed only the internal timings. An offroad is made for less RPM but more bottom torque and most of the time it is set with a lower exhaust port which is easy to raise and with that also a smaller cranktiming. Because the lower RPM it is possible that less strong materials are used, high revs takes a lot of strength of the materials.
But an offroad engine in an onroad car like a Mugen or Serpent gets a shorter manifold which will give automaticly a higher rpm, that will result into a not so bad performing car, it is for sure a right choice on short tracks. On a GT car which is buggy based you will get a longer manifold and so standard buggy performances. But a good buggy engine will perform better than a standard LRP, SH or Force .28 engine and with the right drivestyle it could save fuel as the same theory is used with truggy cars.
Speed is only fiction but a 130+km/h MRS4 is for real.