While cleaning my car the other day I noticed that the big spur gear was damaged, does anyone know what caused this problem and how to solve/stop it from happening again?

A piece of debris possibly got in there, or motor mount screws came loose. Some motors have play in the shaft; not having the proper amount of shims; allowing shaft to move side to side. Line it up (pinion) with spur gear and set the mesh not too tight. The spur/pinion gear mesh should have a smidgin of play.

Looking at the second picture... I would say a piece of dirt got in there and flung back out, as I see only a partial of the spur gear teeth are "chewed up".

I'm also guessing here... that's a nitro RC and no gear cover is used or none available for your vehicle?

There was some dirt and grit when I cleaned it so I got rid of all that and took it out and ended up with this (see photos). No I haven't got a gear cover for the buggy just the main body shell.

I have done that to my spur gears many times, but that was on my electric buggies and truck. Most of the time when I get a spur gear looking like your latest pictures, it's because my motor mount screws loosened up a bit; allowing a good gear mesh to be lost. I don't know anything about nitro RCs and how a motor mounts to a chassis and/or if you can adjust it (which I assume you can if you are able to choose different pinions).

Is it possible to get a metal spur gear for your RC?

The engine and motor mount must've jerked forward when I had a crash the other day which would make sense. I've tried looking for replacement spur gears for my car but can't even find a plastic one at the moment. A metal one would be better I think. Thanks for the explanation.