Maybe a bolt out would work:
Dangerously close to an easy out, I know. Finding the right size may be difficult. The ones in the picture are Irwin Bolt Grips, but Sears has something similar.
Good Luck,
~john
I bought the set at Sears, and I use it more than the other "you must have this tool to work on old Honda's," the impact driver. I use these to remove just about any frozen bolt/screw, rounded head, broken studs, you name it.
Hammer the appropriate size on the end of a broken shaft, then turn with a wrench. Bang on it with a hammer at the same time to create impact vibration. Pieces I have soaked with PB Blaster, and heated with a butane torch still wouldn't budge, but with this tool they crack loose pretty easy.
I try and spread the good word every opportunity I get. These work GREAT! You NEED this tool!