My motor is 72 pounds, not including the encoder and shroud (negligible). Anything lighter than that is preferable. The 3HP 56C washdown motor I found on eBay is 65lbs, and the 2HP is about 45lbs. That's kind of the target as those would be the easiest motors to install since the bolt pattern is the same as my current motor.
But the real issue with my current is motor the outside diameter. The 145TC/56C face dimension fits - but the cast iron motor housing OD is significantly larger than a sheetmetal housing with the same mounting dimensions. The cast iron cooling fins and non-removable base mean the actual OD is closer to the next frame size larger.
This extra-large OD is what is encroaching close enough to the spindle that an air cylinder or air/hydraulic power drawbar assembly won't fit directly over the spindle.
A sheetmetal motor in 56C or 145TC might work, but I don't know for certain. I would have pull my motor and make a mockup to see just how close a sheetmetal motor is to the spindle.
Photo shows the fundamental issue. Even if I moved an oil distribution manifold and shoved the motor back a bit, I'll only gain 0.5"
This is why I'm thinking 90/100mm servo.