That reverse tap sounds like a good idea, the trick being to make sure it's indexed with the threads before starting the thread clean-up.
The only advice I have on the subject is to never disturb the spark plug unless you have to - I've had them run fifteen years in mowers and just as long in 2-cycle tools.
If you do take it out, be sure to give it two full turns by hand after getting it started in the threads before putting a tool on it.
If you're afraid it will get stuck if you don't take it out, just loosen it two turns then retighten without removal.
Gets my goat - the first thing somebody wants to do is to pull the plug. 999 times out of 1000, the issue is fuel, not fire. With today's unleaded gas and lean-burn engines, along with the use of fuel stabilizer, a spark plug may last the life of the engine.

The plug that was in this was the 1st ever plug in my 47 years on this planet that absolutely DIED on me. Started right up when I put gas in it at my detached garage. I store them with the tank and carb bowl dry. Anyways, 5 minutes later I'd filled the tank on my tree sprayer with water and Captan...and...NADA. The only spare plug I had on hand was a slightly hotter plug with the same threads, reach, etc. Started right up. Go figure. I ran it twice before I decided to reinstall the newly bought correct heat range plug. Then the problem. This engine had been cross threaded when I bought it used for a steal of a deal and I had had it recut at a shop successfully. I have always been very careful with it knowing how easy...but I must've had a brain fart. Actually, very hard to tell how bad it is as I am unwilling to force it...it might be only slightly screwed up.

Anyways, first ever spark plug that literally died on me. I put it in another engine and Poof...nothing. I STILL have a plug in my Cub 129 from when I put the short block in it...14? years ago. Hard plug to find H10C or something. At least its not cross threaded. I ordered the back tap ($48 from tooltopia) and in about 5 days I plan to see if it was worth the $. I'll try to remember to visit to crow or to cry about the results...

Couldn't you have just pulled the head, tapped from the inside out and replace the head gasket? Gaskets are about 6 bucks...

On a GC the head is not removable. Crankcase is split on diagonal and cylinder and head are unit assy.

However, if you take it apart that far, with muffler out of the way, etc., you should be able to run a tap down the hole and chase the threads.

Well, I was tickled with it arriving yesterday. They were predicting 5 days...

Anyways, rather a simple tool (I was worried it would be complex) with not a lot of instructions. $48 + the shipping where I found it. I pulled the cross threaded plug, shined a borelight into the hole and rotated the piston away from where the plug seats. To start, apply a 'gob' of grease to the bottom of the tool to catch any errant metal that the tool throws off. Insert the tool, turn the knob to expand its threads to engage the good ones at the bottom of plugs reach. Once it has snugly engaged the threads, wind it out. I cleaned up any loose grease and for good measure, inserted it again and repeated the process. The only dicey parts was reinserting the new plug. I was VERY careful to only turn it all the way in by hand. Once it hit the seat, tighten as usual torque specs suggest.

I'll admit ~$55 for the tool with shipping is a LOT for such a little tool, but I am back in business with this small engine. And to compare it with having it out-of-service for 3 weeks plus the cost of a helicore... no contest.

Anyone interested in this product? I'd give it a hearty endorsement. It's called a "KD back-tap internal spark plug rethread tool" Mine was 14mm to match the plug thread in my honda GC engine. I think they come in all plug sizes. And Made in the good Old U S of A!

Thanks to those that followed along and offered help and suggestions.