And not all motorbikes are without a reverse. I'm not sure, but I think Honda Goldwings have some sort of reverse current going to the starter to back them up.
Uhm, no... it's completely the opposite: usually the engine and the gearbox are completely different parts. Take one series of road car (e.g. my trusty '92 Polo): You can have different engines for one gearbox (e.g. the same 5-speed gearbox for the 55hp, 75hp and maybe the 130hp one) or different gearboxes for the same engine (e.g. the 55hp engine can have a 4-speed or 5-speed gearbox)
iirc it's a Honda CBR600 engine in the MRT5, although the later cars might change engines to take advantage of equivalency standards as they arise, with a teeny little turbo and a 20mm restrictor after the throttle (but before the turbo).
much to complicated, just take the engine, turn it around by 180° (z-axis, everything else isn't very useful ) and reconnect the driveshafts. Now you have 6 reverse gears