Well I think there are a different set of rules (maybe guidelines is a better word) that apply to the folk music world. I do sometimes wonder why we get all these commercial remakes of old pop songs, remakes of old movies, or sequels to movies, especially when often the remake isn't as good as the original. But building on, reinterpreting, and adapting has always been part of folk music and I don't see anything wrong with it. If the tune/lyrics are traditional there are no legal issues involved. I mean, how many traditional songs have been sung to the same tune as "The Star of the County Down" or "Tramps and Hawkers". I think it would be pointless to try to slavishly imitate source singers such as Sheila Stewart or Texas Gladden - perfectly okay to use their version of a song as inspiration and put your own stamp on it. One of my great delights is in listening to different people do different versions of my favorite traditional songs.