Child's collection cites very few tunes that were associated with the ballads. If there is no tune, then why not compose one to fit? In my opinion it is much better to sing a ballad rather than recite it.
It is much less likely that you would need to compose a tune for one of the ballads that was collected by Sharp, as a tune would be there ready for the singing, along with the words.In fact, there are often more tunes than complete texts in Sharp's collection, so you are more likely to be looking to create a composite text than a tune.
I can understand the creative urge to compose, but I would suggest that you try out all the tunes that are available first before rejecting them in favour of your own. After all, if traditional tunes were OK for Haydn, Beethoven and Benjamin Britten just to name 3 composers who arranged English or Scottish folk songs, I think there may well be something to be said for the tunes themselves.
Mary