If your still running the stock motor and esc I would run 12t for the pinion if your running larger tires for your mt conversion
You should be looking into a better esc/motor as I see you are already having problems with the vxl setup
I would look into the 3800kv or 2400kv castle motor or something similar and a good esc

And about the temps you never want to go above 180 degrees on either the esc or motor and make sure your mesh is perfect, just a tiny bit of play between the spur gear and pinion
A tight mesh will definitely heat things up