If it's an original to the round dial 10ee, and in good condition, I would be willing to pay several hundred dollars.
If you are VFD'ing, your one need not be limited to Round Dial.
Any gearbox will need fab of mount-up to AC motor.
All the older one gains you is keyway'ed input shaft rather than splined input shaft, and that can - and has - been dealt with by several PM members already.
Further - as you have to fab mounts REGARDLESS - you are not even limited to OEM Monarch gearbox - nor even GEARS - at all.
An AC motor WILL need some help with reduction ratios 'of some kind'. They are at their best, base RPM and ABOVE, go short on torque, smoothness, and thermal-budget when the attempt is made to run them really slow, even if the VFD is willing to try.
A DC motor is the opposite. At its best base RPM and BELOW, struggles to regulate RPM -> SFM -> cubic removal rate decently when in 'Field Weakened' mode at anything but light loads.
That said, the issue is less about how to get 'enough' ratio reduction, and more about how to BYPASS any noisy Reeves-stye drive, or the 'cogging' effect of gears or PIV-style drive, so as to not 'watermark' fine work from their vibration input.
Monarch's gearbox does that "bypass" trick rather well, and in a small space. "Obviously". It was, after all, purpose-built for the application.
DIY or adaptation reduction ratio equivalents are not hard. They just need about 2 or 3 times as much
space when built-up from diverse stock components. Parallel-shaft IS hard, "inline" shaft, much harder yet.
Even so, at the end of the day, it is that 'bypass' or 'direct drive' feature that is hardest of all to find a practical alternative to.
Not an academic exercise. I spent a good deal of time looking at Dodge and other reduction gears, Polyvee/Microvee double-reduction belt sets, Baruffaldi and other clutches.
Two-stage belts, twin electrically-operated clutches had the most promise, as the ratio could also be selected on-the-fly, and could be expected to NOT 'watermark' the work, nor be overly inefficient, fussy of adjustment, nor noisy.