"I am looking for songs that are generations old, and have been used and reused for songs across those generations."

From what I have seen, I don't think that's possible, Jed. There are too many decades where nobody was paying any attention to the music of ordinary people.

Sure, we have some collections, and later there were recordings, but I don't think there's enough info to do what you hope for.

For one thing, we have a lot more lyrics than tunes written down.

That said, you might look into the tune for "Lambs in the Green Fields." I don't know how many generations it spans, but it's been a popular tune.