I'm reminded of the Kipper Family's lyric, "She had a dark and a roving eye, and another one quite similar."

I think "breast" referring to a single fleshy thoracic protrusion is a fairly modern idea. Breast used to be singular, referring to the chest of a man or woman.

There are many good old fashioned English words for what we now call "breasts", but they are deemed to be less dignified terms. (Tits, dugs, paps, etc.)

Willy probably became a euphemism for penis long after the folk songs were written. Again, I'm reminded of a literary allusion: if Jane Austen had her time again, the protagonist of Mansfield Park would probably not have been called Fanny.