After doing search for aftermarket parts, I see the plastic motor mount is a part of the chassis. So I highly doubt someone being able to find a metal motor mount. Now, if you do find the motor cam loosening constantly, the only thing I can suggest to do is clean the motor mount, and the groove in the cam. One last possible step would be to use hairspray (applied with Q-tip, or similar applicator) on the motor mount/cam when setting the spur/pinion gear mesh. The hairspray isn't used to be a permanent bonding agent, just used as an extra gripping agent to prevent motor from shifting.