Since such ballads are typically sung by single persons in a rather rhetorical style, there is often no clear metre at all. The length of a note varies not only from singer to singer, but even from verse to verse. Thus, any sheet music amounts to an arrangement. I have seen the song printed in 3/4 time (with some triplets), which makes sense.

Mudcat has many threads discussing the question whether modal songs can be harmonized at all. I do think so, but seventh chords should be taboo, since they belong to a different style.