Hi mjzrpr
I have the same task as you, namely to retrofit a Cincom Citizen L20 machine (swisstype) . I also chose Mach4 because I have a lot of options for modifying the software.
I will use two PCs, and I will make the synchronization commands through macros.
Do you want to keep the electronics of the machine, or do you put a new one? Mine had the electronics from Mitsubishi and I will replace it with the one from DMM.
These machines have three types of synchronization, Spindle synchronization (rpm), axis synchronization and axis and spindle synchronization.
The problem with this retrofit is that not being a dual control you can't have a full closed loop feedback like the original.
If the chuck closure is hydraulic for you, you get rid of it easier than me, because mine it's also on the servomotors and it can be adjusted from the software, meaning almost two other axes torque controlled.
I will use a single screen with an input from an HDMI Multi viewer, which will receive the signal from the two PCs.
The controllers I will use for the machine will be two PoKeys57CNC with 5 PoExtBusOC16 and one PoKeys57U as external PLC.
It's a complicated project, but interesting!