Looks like Walthers has several re-powering kits that should do the job. . Thanks Bill
I had a loco whose motor / flywheels were out of balance.
What I did was hold one flywheel and twist the other one some. Maybe 1/8 turn. (The flywheels will slip on the motor shaft some.) Then put power on it and see if the vibration goes away. If not, try again. Make sure you twist them in the same direction each time. What you are doing is balancing them with each other.
I got mine smoothed out that way. It just takes some time. I didn’t even have to take the motor assembly out of the frame, but let it run around on the track.