There was a few of these very long scripts, written in the 70's. At the top was "Here be Dragons".
It felt cool after I figured out what interest calculations it was performing, I ended up leaving a comment "Dragon Slayer was Here" at the end of the script.
There are still younger guys in our banks who COBOL, but the domain expertise sits with the much older guys. It's lately common for the older guys who go on pension to come back as "independent consultants". They earn a lot of course.
Just find one of the experts that are willing to answer whatever question you have on the legacy COBOL code
Some doddling around with an (almost certainly illicitly acquired) copy of IBM Z/OS on an emulator almost proved to me that no free resources exist.
I'll ask around with my mainframe vets and get back to you. There has to be something out there.