The bolt you were talking about is indeed the correct one, but the head size of it is 7/16". It is not the bolt on the fuel tank, it is the bolt on the left side of the pull start. The head size of the bolt in the middle of the fuel tank is 1/2".
Well, the fuel tank one
was the only bolt for which you had mentioned any dimension. And that its matching hole is stripped too, so it will also need to be fixed in similar manner...
Righto then, the original starter bolt will have 1/4"-20TPI UNC thread, and M6x1.0 is the ISO equivalent to that. It may be possible to go up to M8x1.25 here, but this will likely also require that the starter housing hole be drilled out, to ~8.5mm.
Where these procedures [coil insert fitting or just re-tapping] get tricky is when the holes are blind-ended, as at least one of your stripped threads is. In this situation, it's rarely possible to get by with a single tap for each thread size, be that for a standard thread, or the 'special' taps required for the inserts.
Hand threading taps are classed into three basic types, according to the amount of 'leade', or taper, ground on their cutting end. In order of use, these are taper, intermediate, and bottoming taps. For tapping blind holes, the bottoming tap and at least one of the others will be needed.
Sets of three, one of each, are available for standard threads, for just this reason. Buying them may not be worthwhile, for your engine. Sometimes 'Neanderthal mechanic' damage just isn't economically repairable, unfortunately.
The bolt holding the automatic choke cover on has a 1/4" head.
Yup, but as I've implied above, we need the diameter of the
thread. To vernier caliper accuracy, at least; inch or millimetre units doesn't matter. We can convert!
