Well, some of the weird grammar is dialect, and some is in there to make things rhyme. Chances are, if the song is well known, the bad grammar version is the way everyone knows it and expects you to sing it. If you can slip a change in gracefully, it can work, but if it's too obvious, it won't.

Funny, with Shoals of Herring, I don't recall anyone singing "was" - it's "were" in my memory. I don't know if everyone I've heard has changed it, or whether I did the editing in my mind.