[Michael]

On bikes I have had it was no problem fitting different sprockets. The same sprockets fit a range of bikes, though probably not all. My guess is a dealer, or an engineer will have something that fits, or that can be made to fit. Just ask around.

Here is a website that will help you choose suitable gearing; the CBR250R is on it already:

Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator