Once built an NSR100 using an NSR150 as a base! It was to sneak past some regulations in a country far away.

Sleeved the cylinder, recut the head, and used a piston from an 80cc or something. Can't remember.

It was actually ok However, matey used it for the transport office tests only and then threw on the spare 150 top end.

I seem to recal that at 100cc with totally the wrong pipe and carb it was still good for over 100 kmh.