You can ride any bike under 260ml except all the following (performance 2 strokes):
All models with the exception of:
Suzuki RGV250
Kawasaki KR250 (KR-1 and KR1s models)
Honda NSR250
Yamaha TZR250
Aprila RS250
But you can also ride bikes under 660ml (or cc) which do not exceed a power to weight ratio of 150 kilowatts per tonne.