120 Spindle

The original motor
The factory motor is an IEC100 frame size foot-mount motor of 3.5HP max. It is DC, 50-6000 RPM. It requires separate and variable field (73-310V) and armature (73-400V) supplies; the armature voltage is varied with field at max for speed control between 50-1700 RPM, then field voltage is lowered with armature voltage at max for speeds from 1700-6000 RPM. This approach is called field weakening and is common on DC spindle motors. (See motor plate.) For retrofitters, this presents the following problems: Pulley sizing provides a 20% speed reduction from motor to spindle.
 * DC drives are expensive new and less common in the used market
 * To be able to supply 400VDC output the drive would require higher than 220VAC input (in other words, it won't run on house current)
 * You need a DC drive that can do field weakening (this may eliminate some older cheaply available ones)

Keeping the original Siemens motor
It is probably possible to continue to use the existing DC motor with a retrofitted DC drive, making note of the issues above. It would probably require a higher supply voltage than 220VAC, and an expensive drive, but spindle speed control would be very good. The field weakening approach roughly means the motor supplies constant torque from 50-1700RPM (excellent for a lathe spindle) and constant horsepower from 1700-6000 RPM.

Retrofitting an AC motor
The issues with DC motor control are most easily circumvented by using an AC motor. Typically this means a 3-phase inverter-duty motor of 3-5hp, paired with an inverter drive that can take single-phase 220VAC (house current) input. Suitable offshore-made drives and motors can even be found new fairly cheaply. Most known retrofits take this approach. Speed control and low-speed torque will be better with a sensorless-vector drive, and best with a drive that can take an encoder pulse from the spindle. It will be worst with a simple V/Hz drive.

Retrofitting a servo motor as spindle
It's possible to use a large servo motor as a spindle, supplying the servo drive with step and direction pulses to control the spindle speed. This is common in some machine tools to get position control of the spindle for powered tooling or rigid tapping, but that advantage is lost on a belt drive machine. But if might be possible to find a servo and drive that can supply 3-5hp at the right RPM range that takes 220VAC input, possibly giving better performance than an AC motor.

Mounting Considerations
Ideally, we would just use an IEC100 size motor for our retrofit (same dimensions as the old one). They seem to be rare however and not available from the usual cheap sources (factorymation, automation direct). We therefore have to be sure of three things before buying a motor to retrofit:
 * that it will physically fit inside the lathe
 * that it can foot-mount to the hinge pin inside the lathe (even if new holes have to be drilled)
 * that the sheave (motor pulley) can be made to fit the shaft

See list of known-working retrofit motors.