The skin likely rotted away long ago. I think I’ve seen a photo of the eyes in an exhibit somewhere. (Propstore has had a few face castings and internal pieces in their collection or auctions.) I’m sure there must be some photographic reference materials in the Lucasfilm archives.

JEDIT: Photo taken at Chicago Museum of Science and Industry in 1996.

If any of the original Jabba puppeteers are still around, and they’re brought into the production, that would be the icing on the cake.