The tool that rethreads threads from napa looks like a sleeve with a bolt head on the back and a split thread on the front. it collapses sort of then you put it in the hold. You twist the bolt head on the back and it spreads to the plug size and then you back it out slow with the bolt head. It's at napa in thier tool catalog. I think you might find it online if you searched.

If you heli-coil it, you drill it slightly bigger, then you tap the hole slightly bigger, then the "heli-coil" is inserted into the new thread you made with the tap. However the "heli-coil" is the same thread size as your old plug so no you use the same plugs.
Do a search on heli-coil if you still don't get it. You kinda almost have to do it to understand how it works.