Paul

HPGL is a language that used to be used to speak to printers via line drivers (microchips from days of yor). It lent itself well to basic machine control and grew up a bit. Post as much info as you can about your machine, links to on line manuals etc. You may be able to use Mach3, EMC2 or similar to control your machine. This means that you will be able to use a more common standard that is more structured in the way of G code. This will be much easier to support. If you have encoders on the motors they could be DC motors or servos. Don't worry too much about it i'm sure with a bit of input from the community here you can be up and running :-).
Have a good look around the control unit and find out any board info/ markings. Let us know what power supply voltages are etc. As much info as you can gather. People convert huge industrial machines to run on G code from proprietary systems so getting tis alive should be the same process. Look up biesse machines running on the above and you will see what i mean.