Do you have the project files? Or at least a printout of of the program? Preferably commented and with symbolic list. Do you have other documentation like electrical drawings, sequence diagram, instruction manual, pictures etc.?
What kind of equipment is it? The 511 was a typical OEM choice 30 years ago. Mainly to replace relay logic. Additional custom electronics for analog signals, high speed counters, drive control etc. was also typical for this period. Do you plan to replace more than just the PLC?
I have converted several PLC programs between multiple platforms. If you can’t find anyone else closer by (I live in Norway, Europe) I would be happy to help you.