If you have any old motors with the flat, use one of those? If not, we have had success with a drive motor by crimping the motor shaft (by which I mean hitting it with something very hard and sharp to misshape it), repressing the gear onto the now slightly distorted (be careful not to bend) shaft, and using a decent two-part metal glue. Can't remember how many fights our fix did, 10 or so, with no problems. :]