Don't know about the existing electronics, and don't know MUCH about plasma machines. If the original system ran from a PMDX-122, then it SHOULD run from the PMDX-122 connected to a PMDX-411 (and Mach4). The only caveat is that plasma machines generate LOTS of electrical noise (about the only thing I do know about them), and that MAY cause issues with the USB connection to the 411. It all depends on cable routing, shielding and grounding techniques used with the machine.
PMDX does have the PMX-424 with an isolated USB interface that may help reduce susceptibility to electrical noise. But the 424 would replace the PMDX-122, so you would have to do some reverse engineering to figure out what motor step/dir signals and limit switch, etc. inputs go where. Though, you will need to know that even if you keep the PMDX-122 and use a PMDX-411 so that you can create your Mach4 profile.