If you can get a hold of the bolt with the vise grips that is an option. If it doesn't come out with the vise grips you may want to try heating it up with a propane torch. The propane will heat around the bolt and possibly break it loose without ruining the cylinder head or manifold whichever it goes into. If this does not work you will have to drill it out and retap the threads into it. A self tapping screw is not a good idea. They are usually coarse thread and going into aluminum could cause the hole to become to big. Make sure you do not drill to far in or you could cause more damage. If you have access to easy outs they may be able to remove the bolt after drilling. The way they work is after a hole is drilled into the bolt the easy out is tapped into the bolt and then you turn it counterclockwise to remove it. Most times the easy out will grab the bolt from the inside and twist it out.

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