The Big Mill I've got had the same controller and probably similar type motors and drives. Thou i think the Bridge port size machines used slightly smaller motors.?
The motors will most likely be badged up Analim but they are actually made by SEM and I'd guess around 3.5Nm -4.5Nm (30-40 IN-LBS) 4000rpm Max voltage 120-140V and will running at 100-120Vdc. Often they will have a voltage rating of around 36v per 1000rpm so 110Vdc will give 3100rpm.
They could have Resolvers or Encoders with Tacho but most likely Encoders but they will be 5 wire single end encoders and around 1000ppr.

The drives will be the same Badged Analim but the will most likely be made by Glentek. They will most likely be 200A drives if the larger motors or 160A if the smaller ones.

The simplest and cheapest solution would be to re-use the DC motor but replace the Resolvers/Tacho with differential encoders and then use DC drives with Step + Dir.
CNC drive makes 160VDC 35A drives with Step & Dir inputs which will work nicely with UC300. They also have tuning software for the drives.
If the motors have Single-ended Encoders fitted you can also get a differential line driver from CNCdrive so can use with there DC drives which only accept differential signals.

Or maybe go with the ET10 from PV Automation and run it on MYCNC and keep it fully closed loop with original drives etc. Though I think you'll buy the drives and UC300 cheaper than the ET10 and does a machine like this really need Fully closed-loop and all the tuning hassles that can come with it and old Analog drives.?

However, If it was me I'd be fitting Chinese 1.2Kw AC Servos with incremental encoders which will be more than enough for this machine and using something like ET6 or ET7 depending on I/O needs.
I wouldn't go with a UC300 with AC servos as it's limited to 100Khz so you'll need to electronically gear it in the drive to get the speed you'll require and that means you are wasting the resolution of the encoders which is what gives you the smoothness and accuracy. The UC400 would be a slightly better choice but still, you'll need some electronic gearing to get full rpm out of the motors because quadrature 2500ppr encoders will require 500Khz for full rated 3000rpm of typical AC servo.