It sounds like the wire that is not sparking is grounded to the armature plate.
The spark plug wires on that mag plate are loaded against tabs that are buried in the tar.
You can unscrew the baklite ferrule that is capturing the offending plug wire and gently pull out the wire for replacement. As long as your replacing one wire you should replace them both.
A word of caution, if all is original the wires are just spring loaded against the tabs. if its been monkied with its possible someone long ago soldered the wires to the tabs and pulling on them will break the tabs off the coil.
Good luck.
