looking for advice on retrofitting hydrolic mill

Well, one motor driving multiple axes sounds like it has to have clutches. To use modern controls, you need to drive EACH axis with a dedicated motor. It will make a MUCH more versatile machine.
Jon

As Timo and Jon pointed out, This Old Tony's MAHO 400P was like your Knuth. The machine had an early automation with a single central axis drive motor, and a set of clutches, brakes and angle gearboxes to transmit the drive to the selected axis. Only a single axis could move at a time. No integrated motion possible. No angles, no curves, no contouring, no thread milling etc is possible.

Tony stripped out all those mechanical parts and converted the machine to three modern drives each dedicated to an axis, and independant, allowing full 3D contouring.

My Maho 400E was already a "normal modern" CNC, so I didn't touch the mechanics or electrics of the drive system. Only replaced the controller, both because it was DOA, and because it had the lowest spec processor Phillips ofered (Intel 8086) and was therefore only 2.5D capable. With LinuxCNC it is now 3D capable.