Not to mention its her lyric too. I thought maybe you were gonna say it was a free to use vocal and and lyric you found online. They've been doing it the other way around forever. You topping off beats and tracks

This way presents some problems cause only the lyric and melody is the song...

Yeah, you can do that, ive never heard of being done that way, but if it inspires a nice chord progression or beat, works.

I know Nashville lyricists ave been doing that forever. Take a hit song, and write new lyrics to it, then dont tell your co writer how you did it, and they will likely compose something completely different.

But it locks you in to structure and you know it's singable.

Good luck with it, like to hear the finished track.