I'm with the others. Unless the tree is a danger to people or property, leave it as is and tap it. Eventually one side might split off, but more than likely the rest will still be standing 50 years from now.

If you want to help it out - you can either put a long bolt through it and clamp it tight, or put a couple of turns of cable around the outside and clamp those. The goal is not so much to pull it back together, as to prevent it seperating any further at the top. If the top can't spread any further, the crack won't get any longer. Both solutions are used routinely.