I know it's an old thread, but just in case you're still looking for the spur gear.
Kimbrough Racing makes bunch of different spur gears and their spur gear #118 (32P 50Tooth) and #119 (32P 52Tooth) work perfectly without mods.
You gotta use friction pegs instead of slipper pads, and you can get them from Amazon (Traxxas 4685).
They also make 32P 46Tooth which is supposedly the size of original spur gear, but it's way smaller than the original spur gear.
hope this would help.