It certainly appears that your spindle drive will be easy to fix, because it does run. On the other hand, one does have to have some personal drive to practically own one.
The best way to go in my experience is detail every aspect of the system, after all its over 50yrs old. Otherwise, one can chase odd operation problems for years, knock one down, the next weak circuit, pops up.
This is a good start,
The correct manual for your machine, each machine was a custom order, and they made changes through the years of the module drive.
With the wiring diagrams, make sure every wire "more then 80" is in the proper numbered location, and tight at all locations, often folks trying to fix them get a wires in the wrong location
Diodes, search forum for low cost replacements, you need spares on hand.
Spare Cj3 field tube, or tubes, these can glow and seem OK, but wig out under load. This tube and the diodes are most always the culprit for iffy operation, when the switches and relays are clean.
Main contractor, and forward and reverse switches, the contacts must be kept in good shape, as well as the motor being clean. All other switches and relays
The filter for the cooling fan, the older module drive machines have small fan, so keeping the filter clean is important.
The machine was not last sold because it ran great, it was run until it wouldnt run anymore, now the entire drive needs to be brought back to specs for reliable operation.
A helpful read is the sequence of operation, there are alot of circuits, but each one is simple,