Well today after a lot of faffing about I've cloned the PC that came with the MIRAC onto a Compac DC7600 SFF (small form factor) that I had kicking about. I chose one off the pile that already had the correct moulded front panel incorporating a floppy as well as as a CD/DVD player and loaded it up with DOS 6.22 as I had the original installation disks.
The PC that came with the MIRAC (a Viglen Genie) was running DOS 6.2 and it turns out that MSBACKUP in DOS 6.22 cannot restore disks created under DOS 6.2 without a different compression algorithm (DBLSPACE) being loaded as a system device - fortunately I found a copy and it worked

Now I just need to sort out a second serial port on this PC

- it uses one to talk to the operator panel keyboard and one for the LCB3 CNC driver board. Needs to be a low profile PCI card and i think I've sourced one on eBay arriving Thursday.
The power interlock for the electronics cabinet has been smashed - long square shaft from DIN rail isolator at the back of the cabinet should engage a door mounted handle, and has obviously been slammed shut a few times out of alignment smashing the external activator beyond repair - one on order should arrive Monday.
The open slot trunking in this cabinet is missing much of it's capping cover - again some should arrive Monday