Hello. Try to put the locomotive on track and measure the Amps absorbed at stall (mean to measure with power on, full throttle, and wheels blocked). Use a good protected power source to avoid any trouble. If it's in the range of decoder, is okay. Did you know the type of rotor? It is 3 pole or is 5 pole?
3 pole are not so good for digital, reacting strange at slow speed.I try long time ago with a Bachmann , and discarded immediately due to jack rabbit stile of running.
If you decided to not use this motor, better try to found one new one and replace it. E-bay and another places are full with replacements. Good luck. Electrical machine look very nice.