There are people who have used egg shells to cover cracks, boiled for cleanliness/sterilize, Super Glued. I would do some research and be sure of your steady hand. You can't get the glue on the snail body. The gel superglue is thicker and easier to use, doesn't run as much. Check for sure which kind because the SW people often use it to glue frags.
I have had a couple with cracks and they were alright after awhile, the one stayed closed up in his shell for a few weeks healing. Clean water for sure, maybe a pinch of aquarium salt, or Stress Coat.