If you don't have an older save in your current game with the sheep, maybe try going directly for it in a new game, and then use the save editor to transfer that to your latest save. The quest items should show up, since they are still list as being in container number whatever, and you can make a duplicate of that container.
I tested this in a save where I had given Bertia the sheep containing some quest items, a teleporter pyramid and some books, and after using the save editor to export the sheep in a save where it was empty and import in a save after 'loosing' the quest items, I got everything back. I didn't test using a sheep from a different playthrough.
I tried just adding a sheep's corpse [Corpse ("Gen_Corpse_Heavy"), 3/4 the way down the Generic list], as well, using the Sheep corpse mod template, but that didn't work. I didn't compare the 2 and see if there was just something else I have to change.
