I have never heard of a 'throttle gear'... but first gear is always the biggest, second gear is the next size down. Looks like you burned up first gear. Also looks like you're missing some screws, this could be the reason for the burned up spur gear if you were running it like that. Those are some pretty cheap parts. If you don't feel like pulling the two speed apart, you could just buy the entire center unit.

Is it at all possible that you did not adjust gear mesh right? The best way to do it is squish a piece of receipt paper between the pinion and spur (first gear only for a two speed), tighten the motor down, then pull the paper out. This usually leaves you with a perfect mesh.