The leviathan seems to be a unique sea creature, which I've always pictured as some sort of sea serpent. This is consistent with Yeshayahu 27:1's description of it as a snake (nachash). An answer to this question suggests that it's a fish (and doesn't address Yeshayahu). We know from the midrash that at the end of days the righteous will feed on it, whatever it is.
One could argue that because the righteous will feed on it, it must be a kosher fish. Or if the leviathan is a serpent, one could argue that when God holds this end-of-days banquet He can declare anything He wants to be permitted, culinarily speaking -- the laws of kashrut as we know them don't apply. (I've no idea if such an argument would be, err, kosher.)
Do any sources say if the leviathan is kosher by the laws of the torah?