[SAJ]

Here is a picture of what I did. I used a countersunk cap screw but a simple grub screw is all that is needed, since it is the threads forming a "Scotch key" that hold the parts together and the head holds nothing, should the threads strip. I used 4 screws, and the ring gear is actually moving forward between them (bending) after many thousand miles. I think 6 screws is better.
The ring gear was shrunk on and loctited after cooling. Starter is 6 volt. It still overcame the loctite and shrink fit.
We have had to do 3 other cars recently too. I drilled mine through the starter hole. I had to use a titanium nitride coated drill since a high speed steel one would not cut. The TiN one was just a cheap import but did all 4 holes really easily. It surprises me that the drill goes in very centrally between hardened ring gear and much softer cast iron, without wandering further into the cast iron.
SAJ in NZ