There is trying to glue a piece of eggshell to the snails shell.
You can boil a shell to get any odd bacteria from it.
Cool it down.
Find / make a piece which looks the right size.
Then making sure not to get any on the body of the snail.
Get Cyanoacrylate gel glue.
Take the snail, put a dot of glue on the egg shell to affix to the snails damaged part.
If you have to, you could put the glue around the damaged part, avoiding the crack itself as much as possible.
Chances are if it works and the snail lives awhile longer, then it could eventually break off and you would do it again.
I had a snail live a decent amount of time after a crack. Also had to repair it again because the glued on piece fell off.
All you can do is try.