You can see Robert Penn Warren's long poem THE BALLAD OF BILLIE [sic] POTTS in The Collected Poems of Robert Penn Warren edited by John Burt (Baton Rouge: Louisiana State University Press, 1998), page 81ff.

Many other sources spell the name "Billy"

The poem was written in 1943. It would be hard to turn it into a singable ballad, since it's written in free verse.

There is an account of the Potts story in The Outlaws of Cave-in-Rock: Historical Accounts of the Famous Highwaymen and River Pirates Who Operated in the Pioneer Days upon the Ohio and Mississippi Rivers and over the Natchez Trace by Otto A Rothert (Originally published Cleveland: Arthur H. Clark Co., 1924; reprinted: Carbondale: Southern Illinois University Press, 1996), page 294ff.

However, a Google search of that book doesn't even find the word "ballad".