I still think that within the context of the poem/ballad that 'raven' would suffice.
Stuartpengdantic and anglingbri, I give you that Cigfran seems the main translation, of half a dozen alternatives, nowadays.
I should add that Ravens feature strongly in Welsh legends and mythology (this would surely have prompted Cerys' lyrics?) which feature: "Bran the Blessed, whose name means "raven" or "crow". He is depicted as giant and the King of the Britons in tale known as the Second Branch of the Mabinogi. Several other characters in Welsh mythology share his name, and ravens figure prominently in the 12th or 13th century text The Dream of Rhonabwy, as the army of King Arthur's knight Owain."
They are all Brân 'something'.
There is a story that England will fall if ever the ravens abandon the Tower of London. Brân the Blessed is associated with the Tower of London in the Welsh Triads, which might be the origin of the story"
Raven in Mythology
"As far as the mythology goes, the first confusion arises over the distinction between Crow and Raven, at least on the European side of the Atlantic. The two appear, in many instances, to be interchangeable, and the appearance of one or the other in a story depends as much on which author is transcribing it as it does on story itself. Whereas Matthews gives Brân the raven almost exclusively, Miranda Jane Green ascribes to the God's companion animal either the crow or the raven..."
Carrion Crow: Brân Dyddyn, Brân Syddyn, Cigfran Leiaf, Cigfran Bach...
Hooded Crow: Brân Lwyd, Brân Glan Mor, or Brân Iwerddon (Ireland)
Rook: Ydfrân (barley or corn) or Brân Bigwen (white beak)
Chough (less scarce in Wales): Brân Goesgoch, Brân Arthur (of Arthurian legend).
Images courtesy of Iolo Williams' Welsh language book on birds.