Pete,
Nothing like a stock working machine! Keep it stock until it breaks. It is nice to be able fix it when it breaks, but I would cross that bridge after it breaks, then consider a conversion.
When I added the new servo motors, I simply drilled and tapped new mounting holes into the cast iron using a drill template to get the holes centered. See attached.
As far as the tool changer software, that is easy... Just break it down into small parts, ram out, ram in, draw bar up, draw bar down, ATC Home, ATC Left, ATC Right and Machine Tool Change Height, Spindle Orientation… Once you have these functions in place, you apply the macros from my BP412 or DM4800 project for your tool changer and your done!
JH