This isn't exactly what you need... but it's similar. I had the same problem when I replaced my water pump housing.
http://www.farmshopstore.com/xtdanubostre.html
They make tools that are for removing screws. They are threaded backwards so what you do is drill out the middle of the broken screw tap in the removal tool, and back the screw out. I'm sure you can find them at a local hardware store.