Turn the engine over until the exaust valve on that cyl is fully open(intake closed) then hook a good shop vac up to the tailpipe and tap away. We use this procedure on bikes all the time, just try to hold on to the tang of the helicoil. Or if you can find them, keenserts are a threaded sleeve with no tang to break off, they have drive pins alongside the threads to lock it in place <img src="/forums/images/graemlins/kewl.gif" alt="" />

With enough time and enough money ya can fix anything