Artmin

Personally i would spend some time seeing where the wires go to for your boards before doing anything. At least at a minimum you will get to understand what drives your machine. that board is just a typical bog standard bob, if it could be exchanged over for the line driver part of your HPGL controller then it would do fine. It's a big ask but could you do a vid of your boards going over all the wires at a nice slow pace so that we can see what connects to what?

Not familiar with the software but Mach3 is cheap and EMC2 is free so i would not fret about that too much. It looks like it has interpreters for most types of code so you should be fine and it expects to see a parallel port from the off.