If you've got a pit pro, you'll have 12 and 14. cheapest way is to get a 12" and 14" set of scooter tires. Once you get serious, you'll want 12" front and rear. this will take a bit of mucking around but then you'll hve a good motard setup.
then you have gearing, suspension, brakes, engine etc.etc. Just goes on and on as far as you want to take it. But a good pit bike with good wheels and tires will be a very fun and cheap proposition.
My PitPro125RR was 450 2nd hand and to change the tires over chould have been as cheap as 150 odd bucks, but I bought a 2nd rim for the front and very good tires for it and in the end it'll be about 750 all up.
good enough for 3rd place at the last motard class race in the MMMCI
