For the two speed i used the hpi 17 and 22 tooth threaded gears with the matching clutch bell (stock is 16/21). So far they work great, but I was surprised when i actually noticed a difference in acceleration. I didn't think one tooth would make much of a difference but it really did. I miss the peeling out and stuff. I was also trying it on some high traction baseball field dirt with some newer knobby tires, so that may have had something to do with it. Either that or my tuning was just way off. Really i'm just hoping to get a longer life out of the hpi setup. The stock clutchbell just wears out entirely way too fast.
I know Hpi makes some 52 tooth spur gears for 1st. I think the savage uses them. I've been looking at them, but they look like they'd be hard to retrofit. It also may just be too large around diameter wise...
Either way let us know if you try anything.