[brian_]

I don’t have a donor motorbike yet. First have to see if it possible to make everything.

The system is like that.....
https://youtu.be/oQczm1IPLv4

I believe that is called a CVT (continuously variable transmission). That being said, there are some pros and cons with that style of transmission. Now
This is just my view on that kind of transmission. I am sure that there more than the two types that, but will talk about the two types that I have worked with.
Belt and chain, The belt kind tends to slip which generates Heat if the belt is not sealed from the elements and gets wet, the problem is exacerbated. The chain type that I have seen is mainly used are in car's and have a steel chain.

Yes, that's a CVT of the most common type, with variable-diameter pulleys and a belt. Strangely to me, the rear pulley diameter is the one which is controlled, unlike a typical snowmobile CVT.

Like most scooters, this transmission is enclosed, eliminating problems with water and road dirt.

The belt and variable pulleys could be replaced with a different belt and plain pulleys to provide a more efficient drive system, but stuck at one fixed ratio, if desired for use with an electric motor. The motor would presumably be mounted where the engine currently is, not in this CVT housing; however, the motor could possibly be mounted at the front end, sticking out of the housing.

 

[brian_]

it's the front, the rear is just spring loaded.
what's different to the systems I know (mopeds), is that the clutch is at the rear and not at the front.

Thanks - that makes more sense: they have a centrifugal clutch (strangely at the rear), in addition to the variable pulleys (with the control at the front as it should be). As I recall, snowmobiles just let the belt slip on the input pulley until the engine turns at high enough speed to cause enough clamping force (they even call the pulley control system the "clutch"), and this is much better. That clutch (mounted at either end) is certainly not required - or wanted - with an electric motor. A non-slipping belt-type CVT might work well, keeping the motor in a more useful speed range, although with the right motor it isn't necessary.