I dont know exactly what the tool is called, but i think its like a spark plug tap. it is a reverse tap. you put this tool in then screw it to fit snugly into the threads and then you slowly reverse it out of the spark plug hole, should regroove it. Otherwise you can heli coil it, that is just making the hole bigger and then putting a sleeve in it. Go to sears or napa they will have what you need.
Since you only have two or three threads messed up I would use a thread chaser. Just work the threads from the inside out. Your inner threads should be enough to allow you to tighten your plug. If all or most of the threads were marred then I would remove the head drill and install a heli-coil (sp?) Just take your time with the chaser.
I finally got around to fixing the threads on my snowblower. You guys were right – I went with the thread chaser and it worked perfectly. Only cost me $6 at Sears. Many thanks again to everyone on the forum.