Robert,
Due to the scaled down size of your proposed engine, I would suggest you look more at a 4BT engine rather than a 6BT. They are available up to 140HP which is only slightly less than the original engines on a 44 tonner. There are lots of surplus or re-man engines available for very reasonable prices. Also I would look at diesel-electric as they would not require transmissions, gearing, etc. They would also be period correct since both GE and I-R were building boxcabs during the 1920's-30s.
The biggest issue I have been running into is finding a suitable generator head to power the electrics. Traction motors and controllers are easy to find, but the generator is the hard part. I'll keep digging unless someone knows of a good DC generator head out there.
4BTs are also a good option but they are pricey to the point of costing more than 6BTs right now.
Rob
Edit: Just found a 26KW diesel DC genset. Now to look up if speed controllers can handle variable voltages to operate electric motors.
Edit 2: HOLY COW! Speed controllers are $$$$$$$, motors are about $4000.00 ea, and that is not including all the wire, relays, fuses ect and the genset is not cheap as well. I think this was why I shied away from diesel-electric.