These are little locos good for a couple cars. They'll pull a heavy load, but that overheats the little motor and after that you don't have any torque. After a while, you'll burn out a winding and get a dead spot. The motors are cheap and easy to replace. You can change one in about 20 minutes. Keep the load light, and a new motor will run the loco happily for ages. In the asylum with 5ft curves on the floor, a new motor was content to pull 2 Sierra coaches hour after hour. With 3 Sierra coaches, it overheated.
I bought a pittman motor from North West Short Line with the intent of repowering one of these. The guy at Northwest said, "Just move the worms from one to the other." Whew! I never managed to budge one of those. They are REALLY on there. Somebody told me you can get those off with a torch. Just heat the brass worm hot until it practially drops off.
I've thought about buying some prime mover parts and building half a mikado drive under one. I think I could hide the motor under the cab, or maybe between the cylinders.
Of course, my Bachmann Annie pulls 4 sierra coaches for hours and hours and hours and I've never had to change the motor.