Does the crack penetrate to the tobacco chamber? If not you might look at laying your hands on a block of meerschaum. I recently saw some blocks for sale on the internet, but don't remember where - maybe Ebay. Someone here may know. They are for carve your own pipe aficionados. I would create some meerschaum dust. Using an instant glue in the crack I would work the meerschaum dust into the glue before it sets-up. I would use a slower setting instant glue. You should be able sand the surface with very fine sandpaper or micromesh like they use in making pens. Then you can work/polish some bees wax on the surface. Given that the pipe holds sentimental value for you being your father's, only you can determine if it's worth the effort, or a retirement in your pipe cabinet. The crack was probably from a temperature difference plus a flaw (small air pit) in the meerschaum. I have never cracked one or seen many so figure its rare.