Looks like that middle board also handles the stepper power supply, going by the transformer connections and capacitors. Have you found a wiring diagram for the machine, as it'll make working out exactly what that board does, and what signals it needs to work, far easier. If there's not already one over on the Denford forums, try asking as they're usually pretty helpful.

Is it running VR Milling 2 or 5?
If it was 5, I'd personally leave it until such time you have a reason to change. It may appear a bit 'simplistic' but it just works, and accepts standard G-code like most other controllers.