Personally I think it's acceptable to modify folk songs - adapting them to yourself and your singing style. It's part of the folk process and there are probably no real "definitive" versions out there - there would be many variations already and you've just as much right as anybody else to put your own "twist" to a song. As a matter of fact I have done it inadvertently to some songs I've been singing since childhood. I thought I was faithfully reproducing them but upon hearing the originals again after many years I realized that changes had crept into the lyrics I was singing (minor ones) and that there were slight melody changes as well, that I hadn't set out to do - it just happened. It's a little different if you're dealing with a known author's song. I also think it's okay to change the gender if it's more comfortable for the singer.