Is it the shell that is cracked, or the seat tube?
I've seen this happen with bulge-formed BB shells. You can repair it, but it is a big job. First you need to cut the shell into several sections, then sweat each section off the tubes, clean them all up, and braze in a new shell. If the new shell is different from the old one, you may also need to replace the chainstay bridge. And of course you'll need to repaint.