Assuming that the spindle motor has no problems. You may need to skim and re-groove the commutator and put in new brushes.
The trick is getting all that to adjust at the right times. There is a single knob on the lathe which has the two rheostats or pots on a single shaft, which each have a range during which they do not change, as the other one changes.
Doing this with switched taps -- or even a pair of Variacs -- gets a bit tricky.
Yes -- you could use the two pots on the single shaft to control two power supplies -- with the proper control electronics and SCRs or triacs -- but you are then getting close to the original electronics in the later models. If you have one of the later models with a dead set of electronics, this might be the best way to go. But for a M/G version, I think that making that work as designed is the better approach.
There is also a mechanical tach in the headstock to tell you the RPM you are currently using. (The one at work had that dead, unfortunately, so I never knew the actual speed -- just went for what felt right. :-)
If the motor and generator still work -- or can be made to work fairly easily -- it is easier to go that way.
When you come in to use the lathe, you bend over and hit a button which (IIRC) is near the floor on the tailstock end. You hear the motor generator spin up, and then it is ready to use. Then all you have to do is remember to turn it off when you are done.
Enjoy, DoN.