Could one use four magnets and sensors. Space the magnets and sensors lengthwise on the hub enough so they do not trigger the the other sensors? Yes it would fire on the exhaust stroke but that does not matter. If two opposed cylinders fire a the same time you would only need two sensors? If my scheme is workable you would only need one ignition with four HT leads.

Nice job on the hub. I need to get a nurling tool like that.