I think for all the carving and glueing you'd do to repair it, you may as well put in a new one.
StewMac sells pre-made nuts in common styles, so you'd just have to sand it a bit to make it fit just right, and file the nut slots to the final height. If they don't have one that fits, you would buy a blank and carve it out.
Making a nut from a blank is not technically difficult (you just file and sand on it until it's just like the old one), but it is a bit tedious. Bone is extremely hard stuff. Power tools make the job go faster. Tusq (a man-made material) is a little easier to work with, and gives a very good result. And Tusq doesn't smell bad while you're filing on it--when you file bone, it smells like when the dentist is drilling your tooth!
Ed