Use the parameter solver link in the R/C calculations (in sig). Just enter your desired speed, spur, diff, V, etc, but leave the pinion blank. It will calculate the needed pinion to get that speed...
Well, what are you shooting for speed-wise? And what is the approximate weight?
Truggies have larger tires, which is like adding a tooth or two. And since you're running a 46T spur, that makes for fairly tall gearing for a truggy. So, I'd start with 12-13T pinion and see how things go. Watch temps and adjust accordingly.