I'm certainly not the expert here, but...
It is certainly possible (and fairly easy) to remove their computer and stepper drivers and replace them with a PC and EMC2 or Mach3 and modern stepper controllers, but running the old steppers.
It also might be possible to figure out what signals their computer was sending to their stepper drivers and find a way to coerce a PC to send those same signals to the existing motor drivers. The big question here is, is it worth it? Might, or might not, be a lot of work to figure out how to convert just machine using their system.
On the other hand, maybe someone else on this board might already know how to do it with just a PC.