If the DSP is dead then it should just be a case of ripping out the DSP interface card and replacing with a BOB then connecting via parallel port. If you want USB or Ethernet then you'll also need a Motion control card like the ESS or UC300 etc.

These machines are so simple wiring won't be difficult has there will most likely only be E-stop and drive signal wires to swap over.

Personally with these machines I'd just use it via the parallel port has ESS or Other Motion control card advantages is wasted on them other than allowing connection to PC via other form.
If your just testing the CNC waters so to speak which these cheap machines are great for then parallel port is cheap quick and matches the machines capabiltys easily. Save the money spent on ESS etc and spend it on the next better more capable machine.!