I think it would be very hard to go that route unless you wanted to drill the head through to take the oversize bolts (assuming they could be got anyway with, say, 7/16th BSF shanks and the small head required to get spanner clearance on these engines). When I had one stripped thread in the barrel of mine, it was luckily on a hole that could be equipped with a stud, so I made one with a 7/16th BSF thread into the barrel and standard 3/8th diameter up top, to take a one-off deep nut. It was the rear right-hand-side hole behind the plug. The holes in the centre of the engine wouldn't, I think, allow the use of a stud and nut unless the engine were taken out to put the head on, which is taking things a bit far.
And such solutions aren't pretty, however functional! Helicoils offer a better answer, which I'd have adopted had I known they were OK for these sorts of applications at the time. It was the folk here who clarified the suitability of helicoils then; the advice is still good, and I think certainly the best option if the whole lot need sorting out.