The reverse that some of the GasGas quads use has nothing to do with the transmission. Rather, it is the ignition, which, when the reverse is activated, actually causes the engine to run backwards when started.
This is accompanied with a rev limiter, which keeps the quad from going too fast in reverse.
The concept for this reverse system was particularly easy for me to grasp, because of my prior experience with my second bike as a teenager. The bike was a 1965 "Harley Davidson M-65 Sport"; a small piston port 65cc 2-stroke made in Italy for Harley.
The port timing and ignition timing was so close that I was able to put the bike in gear and push (bump) start it backwards.
I would do this for fun, and stand astride the little bike while it was idling for a bit; until I had an unsuspecting audience. Then I would rev it up and dump the clutch; spinning gravel on whoever was standing in front of me. Damn, I thought that was funny.
I would not want the quad ignition with the reverse on a dirt bike, but I think that such an engine would accept a standard bike ignition. I would also want a bike's black box to go along with any quad engine installed on a bike. If my memory serves me correctly, the case was different on the quads with the reverse ability (on the ignition side).
Reggie is currently building a project race bike, using a counterbalanced '98 EC250 engine, which he is installing into a 2005 GasGas frame.