The above piston from Wiseco, is for 4.030 but there may be stock diameters available
It is a dome but the lower comp height gives it 10.5 to 1 with 67cc chambers.
You could have the top milled slightly to further decrease compression as well as remove material from the combustion chamber on the head to get it right where you want it.
I think any way you slice it, it will be a lot of work.
I have seen advertisements for a fuel treatment that increases your octane significantly, it is a cannister that you put inline with your fuel system and could bump the 98 to over 104 or something like that. You should check that out as well.