There is a lot of information in WINCNC on how to program etc. that would be helpful to you when programming. No it will not write code for you, but will assist with coding and general questions even. I would suggest developing a template to work from and doing your basic editing in WINCNC and using it for timing on your machine. Then transfer to the machine for final proveout. The M20 is before my time but should program similar to the A20 or L20 of today depending on which control it has. I think yours has the mits control and would program like the L20 for the most part with some possible M code changes.
I would push your dealer more since they can most likely do it, but are just more interested in selling you a new machine because of it's vintage.
Do you have any sample code to work from?