JDK
I ran a 21 tooth pinion with the stock 84 tooth spur in stock last time i raced, on a flowing track with high top speed- so try there... you will probably use between 19-23 at your track... only judging by photos as it seems to warrant fairly high top speed right???
try it and see... start from 20 and work up, you should find the right one in there
Hope that helps
Robbie