Motorbike gear ratio question?

I have a motorized bike with a chainsaw motor and i was wondering what gear ratio to use with it. The engine, from a chainsaw, has high rpms (around 1250) and a cintrifugal clutch. I am using 21" tires with the bike as well. Please help me!!!! and if your gonna say that it is a dumb idea dont even bother answering..thanks