Start with electrical pick up!
Tyler,
With locomotives that have the standard electrical pick up for steamers, One rail picking up on the tender, and the other on the locomotive, the best improvement for the buck comes from improved electrical pick up. I use phosphor bronze wire on little pads of PC board which I fasten to the frame with epoxy or Walther's Goo. Picking up from the insulated side of the drivers is the easiest modification. the insulated side of the tender trucks is a little trickier, but well worth it. The old AHM locomotives have decent gears, and their motors were very good for the 1970's; but much better motors are available now.
Getting into the gearing on a steamer can be a real ***** due to quartering issues, so My approach would be to do electrical pick up first, and then go with a good quality motor, On that locomotive a gear reduction motor might make it to slow. that would be acceptable if it was a switcher, but that locomotive looks like a thoroughbred, and should be able to get up and run when given the chance.
Let me know if I can help, or if you would like to use my shops. I have an AHM Genoa that needs the electrical pick up and motor treatment, so perhaps you could follow along.
Bill