Here is an additional 2 cents of observation. . . .
Long ago, I also stripped the threads of the cheap engine I had. Studs were 6mm. I almost threw away the engine, but I decided to try something out of the ordinary. (Kind of like how most of us could describe ourselves.)
I drilled out the head and the cylinder case to accept 5/16 inch fine thread Grade 8 bolts, four inches long as I recall, readily available from Ace Hardware. I used the recommended twist drill for the tap. The drill did break through the thin side wall casting, but not enough to affect finished product. I tightened the bolts to 8 ft-lbs each. The head has not been pulled since and I put well over 1300 miles on that engine, with some threads showing and all.
It is possible to go to larger bolts, but maybe only one size.