if the gear thread still ok, then maybe it's just the gear crack causing the gear to slip.. if that's your case, then all you need is some glue to hold the gear back in position to the shaft...
if your gear teeth all sudah patah, then sorry lah babe... you have to get new gears to replace it...
i'm working on it also coz all my gear no more gigi... maybe can find some at the tamiya shop where they sell "upgrade" kits for the model cars...