I would concentrate on locating documentation with a view to the replacement of the entire PLC. I would not waste any time on trying to locate spares to try and keep this relic up and running.
Back in those days you would always print out the program to paper. If you can find this then you have something to support the creation of the new PLC program. Just be warned that the PLC looks so old, that a program listing probably does not have any symbolic addressing or like that.
I think that you are possibly looking at writing a completely new program from scratch with very little information carrying over from the old PLC. This may not be so difficult unless there is some special know-how hidden in the old code.
I have done a few migration projects in my time, and I have never migrated the code, I have always written new code from scratch using all the functionality and power of modern PLCs. The only reason I have had to look into the old code was for understanding some functionality that otherwise would not be obvious.