Phew! You have an interesting task
If this is an EXCEXTFUN type then you may have a chance. First – copy your source, rename it, and keep it secure.
– Did you check for any versioned copies?
Recreate an EXCEXTFUN and give it the missing functions generation name.
Do you know what the lost fun did? – as a specification, say. Is it trivial or serious, or essential or business critical?
You can consider maintaining it in RPG in future ?
But if you still need to regenerate the lost action diagram, then you need to examine a source list and abstract the functions used by examining the subroutines comments – where all the components are named. You can quickly extract a component list, and you can quickly construct a flow of nested routines by inspecting the interraction between mainline and subroutines.
Now look at the CASE constructs – again, the model components and values are there in plain english in the comments, so for each CASE, reconstruct it with its conditions from the comments.
An EXCEXTFUN has no structure, unlike, say, a DSPFIL, so its mostly a top to bottom flow of control – lucky