Chris,
Pete has pinions, he stocks RW pinions. What are you looking to change it to?
As a guide at york: I run a 10.5T with an 84T spur and 22T pinion.
Somewhere bigger i run 8.5T 84T spur and 23T pinion. I did run that at york but found it wouldn't get to full speed on the straight, so I should've knocked it down a couple of teeth on the pinion. But I couldn't be bothered
.