My 2 cents worth.
For stripped holes other than spark plugs, Helicoils are the way to go.
Because spark plug are removed and replaced so often, the one piece insert, mentioned elsewhere in this thread, with a shoulder to tighten up against is a better solution. I dont remember the brand name of these, but they are readily available at auto parts and hardware stores.
Down sides to helicoils are that the sealing surface between the head and the plug are reduced by the size of the helicoil, the coil can loosen and move in the hole as you change plugs and my all time favorite, which I saw last year, the inner turn of the helicoil extends into the combustion chamber where the head can no longer heat sink it. This coil then gets hot enough to glow like a glow plug and fire the mixture as soon as the transfer ports open.
It did not backfire through the carb because it had reed valves, but it really did a number on all the crankcase seals. I think the only reason it continued to run was it was a two cylinder.
Ron