Chris Henry wrote:

convert to DC drive motor or VFD . I can get a 10 Hp dc 180 volt motor
and controller for about what i would pay for the VFD.


its a monarch 20 x 54 , 1945 vintage

A Monarch of that vintage would have 16 spindle speeds, hardly warrants a vfd.
The best bang for a buck would be a bigger motor pulley & belts to speed up the
spindle. The optional bigger hp Monarchs were usually slower than the standard
models.
10 hp was standard for the 20 inch of that vintage.

Tom