Drew, it varies with skill and weight, but a 10/83 will work at PSGKA, Richland, SIMA and PGP (if they ever let 2-strokes back). You might wind up running up one or down one or two in the long run, but this is a good start point.
As fate would have it, I have an extra used Leopard 10-tooth clutch drum and sprocket and an extra 83-tooth axle sprocket left over from when I used to run a Leopard. I’m in Seattle. PM me if you want to discuss a purchase.
For new parts, Italian Motors (located in Sumas, WA) is your source for all things and knowledge that is Leopard. http://www.italianmotorsusa.com.