Joe, Its the one from the starter set 0-8-0s. You can get it from Lionel and maybe a few of dealers on the forum. I removed the motor from my engine and replaced it with the larger one but I see it on Lionels website.
One caveat, they come with the gear. If you don't need it they are difficult to remove. I had to remove it and put it on the larger motor to mesh with the axle gear.
Pete