If the threads on the spark plug itself are damaged, buying this tool or a new plug will be cheaper than calling me.
If you've gummed up the threads in the plug boss, then it's possible that they can be cleaned up.
Any damage beyond that is a more complicated repair.
If the insert has been pulled out or it's spinning (you can't torque the plug) then the Helicoil must be replaced
DO NOT USE STANDARD HELICOILS FOR SPARK PLUGS ON ENGINES IN CERTIFICATED AIRCRAFT
There are Service Instructions for Lycoming and Continental outlining the proper replacement of damaged spark plug inserts
The FAA does not approve any inserts other than what the manufacturers dictate. (Being Part 91 doesn't help you here.) Spark plug inserts (Helicoils) on aircraft engines are unique, oversized, and require specialized tooling to install.
AIRPLANES HAVE CRASHED AS A RESULT OF USING STANDARD HELICOIL INSERTS.
Unfortunately, the tooling required for this repair is so expensive and requested so rarely it isn't economically feasible for me to provide this service.
Getting your plane (or at least the cylinder) to an engine shop will be your best option.
If you can come up with a way to borrow the tooling from an engine shop, I can perform the repair for $150 on most engines.