On a lark I ordered these when I spotted them on eBay looking for a different motor.
3pcs 16800RPM DC 6V N20 Micro Motor for RC Model Airplane Helicopter
They are identical in appearance and fit to MTH smoke motors. I have no reading on the reliability of these at this point, but they will replace many Lionel or MTH smoke motors if you feel adventurous. At $4.18 total for three of them, the price is right.