I would first try carefully threading a new plug in place and see if it will torque to 13 ft-lbs. If you do have to resort to one of the heli coil kits, PULL THE HEAD to do it. Don't let the chips in the engine.
To prevent this from happening in the future, when changing plugs ALWAYS:
o always, Always, ALWAYS remove plugs when the engine is COLD! You can probably get by with an hour of cool down time, but overnight is MUCH better.
o Blow out spark plug wells with compressed air before removing plugs.
o If you choose to use anti sieze, ONLY use Copper based and use VERY small quantity on the THREADS ONLY. Do not allow it to get on the plug below the threads.
o Torque the new plugs to 13 foot pounds! Before aluminum head engines, I NEVER used a torque wrench on spark plugs, but that was then, this is now!
Good luck.