OK some other 'points' to consider Is it definately an ignition issue...use a timing strobe?
Could it be weak fuel mix ie carb jets etc is blocked with crud especially if it has been sitting a long time, try easy start spray does it run better?
An interesting issue Im looking at is distributor phasing ie does the spark happen when distributor cap post is centrally located with the rotor arm? check where arching happens on the rotor arm ....it should be spread across the edge of rotor arm not on the training/leading edge.....
