You can try some penetrating oil like kroil and let it soak. Hopefully you can free It up.
Repair shaft is available
...link... For broken studs I like to use left hand twist bits. Step drill starting will a small size and work up.
It will take a bit of effort.
http://www.pbase.com/dennygraham/1950_manifolds&page=allTry posting in the parts wanted section if the forum if you chose to replace. Make sure you identify what year Model engine you have.
