How do you register a 90cc as a 50cc?

[trickstar pit]

Hi there. Ive got my cbt test comming up at the end of june, so im looking for a road bike. Ive decided that I want a honda c50, but they are hard to come by because people bought the more powerfull c90 instead.

So my question to you is, could I buy a c90, then register it as a c50 (aka 50cc)?

Ive read somewere that you need to change the engine number and the engine capacity of the new engine on the v5, then send it off. Is it really that simple?

Any help / info will be much appreciated.

Thanks!