the spark plug helicoil set comes with a stepped down tap with 2 sizes on it...the first step on the tap is the same size as a spark plug so using whatever is left of your threads it will start the tap straight...the second step to the tap is the bigger size to thread the hole for the insert....now, technically, yes the by the book way is to remove the head for the repair but in the real world it can be done without removing the head, you just need to be VERY careful!!! like i said...use plenty of grease on the tap to trap the filings and go slowly a little bit at a time once the bigger part of the tap grabs hold....also remove the tap every few turns to wipe it off and apply fresh grease...ive done this many times over the years and have had no problems at all...like i said its not the by the book way to do it but it does work....up to you and i wouldnt blame you if you removed the head...just thought id let ya know of my experience with this type of problem....good luck!!!!
