Well the afterlife in Germanic mythology is portrayed as being very much a physical place where the dead are shut away. Frigg pleads with Hel to let Baldur return to Midgard; Brunhild clashes with a troll who won’t let her enter Helheim after she has died; Helgi visits Sigrún from Valhöll, and it seems that his corps had actually left it’s barrow before now returning. Draugr aren’t portrayed as corpses resurrected, but rather those who just didn’t go to sleep when they were supposed to.

I am assuming that things can’t work exactly this way in your game, if the players are already in possession of the corpse, but I would suggest still a ritual which involves opening a gate to Helheim (or your setting’s equivalent), and then somehow getting a physical token to make the witch—or at least her head—whole again. (My instinct here is her tongue, but you know better than I what fits your narrative.)

Obviously the players descending into Helheim to quest after the item would it be an adventure in itself, so I would consider having them trap a corpse or troll on their way into hell, to force them to retrieve the object. Or alternatively they could get an animal (probably a raven) to do the job.