If you're familiar enough with how your machine operates and its not too complex, I'd rip that whole system apart and rebuild it with off the shelf components. Micrologix 1000's etc..
You have to pay close attention to the safety systems and ensure they're integrated correctly. If it's too much to take on you could go with a consultant.
Drives have many different control structures. If its run off a green board it'll be analog or digital. You need to look at the board and determine what other signals its receiving, why and how they're being used.
At the minimum there will be a OK TO RUN signal and a start signal.
You'd have to be a lot more specific with what your system is doing and what it consists of to get a better answer.
