When you say "gearbox upgrades", are you meaning gearing? If so, that spur gear may be a little hard to find. It has a hex in it, and most 1:10th scale buggies like this use a slipper and that center hole is round so it can slip. But I'm sure with enough searching, you can get better gearing. The pinion is just a matter of matching pitch of the gears (more than likely it's a 48 pitch that is on there, 24 is too corse and 64 is too fine) so you just need to find one that fits your motor shaft.