You can rebuild them but as far as I know its only with plastic gears again. It would be just as cheap to buy a $30 Hitec 645mg and call it done as it will have much more power and basicly the same speed but with metal gears. i would find a faster servo for the throttle/brake though. Metal gears are not necessary here thought, but are always recomended.
ha ha ha.... not worried about the brake yet. I still can't apply the break without killing the engine. I have adjusted the dang idle screw i don't know at the times.
check and make sure that when you pull the brake, the carb stays open. i know on my savvy, i adjusted the throttle on the remote, and it kept dying until i put the remote back to center, and then it kept running with the brakes on.