Replace a pylon? I doubt many people have done this. They're installed in the hulls at the factory using fixtures to make sure everything is in alignment. Without the fixtures, you'd have a hard time making sure the new pylon was in the right spot, and if you mess it up, you may not be able to get the boat back together.
Supposedly Hobie sells a pylon sleeve that you can purchase that fits inside the pylon to reinforce it. But I think availability of this sleeve is limited.
Your best bet may just be to find a replacement hull.