Be aware that Mach 3 doesn't thread correctly on long threads with the single slot encoder.
Mach 4 is getting there but it's been a long haul and as well as a $200 Mach 4 license you will need an external controller board as Mach 4 cannot exist on it's own like Mach 3.

Linux CNC can thread and is free [ can't use USB breakout boards ] but unless you are conversant with Linux it's a steep learning curve.

I had a look yesterday and found the Conect lathe software. It runs off a PC, what flavour I have no idea, probably DOS and I also found the manual for it.

If you want to go the original route I can scan and upload these