Yogi,
The wiring diagram will be a place to start. With it and the help of the mechanics, you will eventually be able to figure out all the Input and Output addresses. It will not help with the PLC internal addresses, which the program will have but will not identify as to what they do.
Uploading the old program would be the easiest part. The figuring out how it operates will depend on your knowledge, your experience with similar equipment, and how fast you can learn new subjects.
To be honest, it sounds to me like you want to go the "challenge" route of rewriting the old program, as opposed to the more logical and potentially less-personally-rewarding route of finding the old copy. Possibly you are thinking that the only copy available would be inside your company. No, it could be anywhere in the world, and you must be willing to do what it takes to find it.
I once needed to track down the wiring diagram for a 50-year-old car crusher, so that it could be put back into service after many years sitting idle. The company had been out of business for 20 years, but after many phone calls, I found the guy who built the machine, now working at a different place. He did not have the wiring diagram, but he could remember exactly what each wire did, so I was able to redraw the schematic by probing his memory on the phone!