Maybe the way to go would be to gear up the magneto to twice it's normal speed and feed it into an 8 cylinder distributor, the tractor magneto may already have it's own 4 cylinder distributor built in.. Purely a mechanical solution.
I have only had dealings with the Minimag 'Falcon' magneto, which is only for a single cylinder, but could easily be up speeded to drive two or four plugs, as long as a distributor is used to send the power to the correct plug.
If a spark source is connected to 2 cylinders one in compression and the other into exhaust the spark will strike on the side with lower pressure. In other words the wrong cylinder.