If your existing control box has a parallel port connector (i.e. DB-25 or 25-pin "D" connector), then the PMDX-411 would be the quickest and easiest PMDX solution. This plugs into a USB port on your PC and a parallel port connector on your controller.
That is presuming that the step and direction pins on your controller are on parallel port pins 2-9 (the parallel port "data" pins). If you can post the pin-out for yoru controller's parallel port connector, or alternatively your Mach3 XML file, I can take a look.