In the mean time, here's a suggestion from a ship modeler who used to work in a hobby shop. Think about putting the motor in the tender, and using its wheels to pickup the current and push the locomotive. Might be a lot easier - and you wouldn't have to worry about the pistons, drive rods, etc.
A good idea, but there are problems with the torque reaction of the motor that have to be overcome. It allows the locomotive to be filled with as much heavy metal, increasing tractive effort, but the torque problem must be overcome.
OTOH, the tender is nearly the perfect place for a sound decoder and speaker, although, technically, it would be better if the speaker was near the exhaust stack and cylinders - that's the loudest part of a real locomotive, and some people are put off by the sound eminating from the wrong end of the locomotive.
Another resource to consider is the Locomotive Repower group on Yahoo. Pay particular attention to the postings by Ken Clark, and if you converse with him, tell him I sent you and told you to say hi for me.