i spun my firestorm into the kerb clonking the motor which messed up the gear mesh causing spur gear failure ,doesnt seem to take much of a knock to mash things up a bit.
if you order a spur gear its worth getting a slipper pad set as i tore mine when removing slipper pads from the old spur gear but that might be just me being a bit rough ha
dont be daunted though easy job if you take your time and lay your parts out oh and have that manual to hand
learnin to fix stuff is part of the fun in this hobby good luck mate