What are the plugs gapped at? The spark will jump the shortest distance and 3/16 inch is 0.188 inches - much more than the typical plug gap.
Try Autolite 3077 plugs. They look wierd but work well in the "F-Series" tractors. My guess is that spark is not your problem. My 10-20 runs just fine with a weak E4A mag.
These old tractors can be frustrating but you will be grinning eat-to-ear when she runs!