Denford use 24V logic in all thier stuff, so ideally you'd use a BOB/controller that is 24V tolerant.
Basic option is to just replace the main control board (the board with the serial connector plugs, which sits on top/plugs into the board with the terminals), trace the terminals and solder into the headers on the terminal board.

More advanced option is replace that entire board, with a controller and 3 new stepper drivers.

For 24V controller, the three I can think of just now are CS-Lab, KFlop+Kanalog, and PLCM (Zapp sell them). CS-Lab and PLCM would require some form of step to analogue converter for the spindle control board. There may also be an issue with the homing sensors that Denford used (they used to use NAMUR output, but I think they swapped to something a bit more sensible at some point)