It is interesting that the belts on this loco degraded. I have a PRR 2-8-0 and a USRA (PRR) 0-6-0 from Sunset with the original belt drives. I think they predated the 70 tonner by some years. Both still run very well and very smooth with no slippage or anything. I also have a newer 2-10-2 that has that belt drive and it runs fine.
Based on that, I would seriously consider just replacing the belts and using the rest of the drive train as is. I guess if you really want to replace the motor, replace with one the same size.
these are Just my thoughts about the drive.
I hope this meets the paragraph request properly!