Welcome. If its your 1st attempt at tinkering with cars, bike engine into a 126 is a BIG job, Fixing it in, no problem making it actually power the wheels thats the problem.
Get a 126 and just play with it before stripping it down to do a rebuild, or all you will have is an empty shell and it will be sitting in the garden until the end of time.
I could be wrong though.
Welcome
Hehe, yeah, so I'm not sure whether I WILL do it.
The best way I think is to hook up some kind of chain drive, I've come across this solution before when putting bike engines in cars. Ideally, I'd want to keep the motorbike engine, as it's sequential, and it wouldl likely have synchromesh on all of the gears, unlike the 126 gearbox. I don't know if it would be up to the job though, as motorbikes are a lot lighter.
I think I'll get the 126 first, as you said, before I try anything difficult, haha. I'm sure there'll be people who'll be able to point me in the right direction if I get stuck.
Thanks for the warm welcome.