Possibility of a Retrofit on Alcor 220CNC

The machine has position feedback through Heidenhain glass scales and a 3 axis module with a single output cable

Not sure what you mean with '3 axis module' but the signals from the glass scales may need to be converted from analog to digital signals using Heidenhains 'EXE' modules. Some people have also built their own converters. Or you could also replace the scales of course. Have a look for type plates on the glass scales to be sure.

Ok I see. I checked in the control cabinet and the box i assumed was a Heidenhain EXE for 3 axes was in fact a HRA 110 unit which is taking the 3 handhweel inputs from the front of the machine. The glass scales are of the type LS 303C and have a sine wave output. To use these scales I would need a converter of some kind to create a square ttl wave for linuxCNC, right? In that case it's an easy price comparison between getting 3 converters or just swapping the heidenhain scales for other scales that already output a ttl signal.

In terms of necessary Mesa cards, the description of the 7i97T reads "8 inputs can be used to support up to 4 quadrature MPGs" so that card would take care of up to 5 analog axes, 1 spindle, my 3 handwheels and still have a few inputs and outputs. I would just need another daughter card that has a bunch of i/o pins to control all other function and read the dro scales. That should satisfy all my control needs, or am I missing something?

Does the monitor and keyboard in the control terminal need some different mesa card or can these be connected to the pc? I would like to use at least the heidenhain keyboard, if not also the monitor and that most likely needs some kind of adapting.

The glass scales are of the type LS 303C and have a sine wave output. To use these scales I would need a converter of some kind to create a square ttl wave for linuxCNC, right?

That is correct.

Does the monitor and keyboard in the control terminal need some different mesa card or can these be connected to the pc?

I would expect the monitor to have a standard input (VGA, DVI, HDMI, ...) but that should be easy enough to verify.
The keyboard will likely be more challenging and will very much depend on the electrical interface used to transmit the key press signals. There are components in LinuxCNC to interface matrix keyboards but frankly I'm not very optimistic when it comes to reusing a Heidenhain keyboard.

In terms of necessary Mesa cards, the description of the 7i97T reads "8 inputs can be used to support up to 4 quadrature MPGs" so that card would take care of up to 5 analog axes, 1 spindle, my 3 handwheels and still have a few inputs and outputs. I would just need another daughter card that has a bunch of i/o pins to control all other function and read the dro scales. That should satisfy all my control needs, or am I missing something?

I'll leave this to the experts to answer.

I see so it's probably much, much easier to go out and buy a standard monitor (with tochscreen) and keyboard und just connect those to the base pc. Plus, I think I could sell the Heidenhain components and get a bit of money back that way.