On the original Varney drive doesn't the motor rotate on the trucks? I worry that with a fixed motor with a belt drive would constrain the trucks such that they will not rotate around curves properly. Also you seem to imply that the counter weight is not fixed to the motor shaft, but that it somehow rotates independently, I've never seen that. But then I have exactly 2 Athearn Diesels (a PA-1 and a PB-1) and the counterweights on those units are fixed to the motor shaft. Should be interesting to see what you come up with!