Get an ohm meter & read between the spark plug lead & ground the other lead. Scale set on 5K If you have around 3500 ohms or so look elsewear for the misfire. On the back of the mag plate you will find two screws, tighten them up a little but not too much for throttle lever still works. Pull the flywheel & check your points gap thru all the throttle advance ranges. If the plate has wear the points gap may change causing misfire. It may be a dirty idle jet screw not letting fuel pass equal too causing a misfire. Those are tiny jet holes & clog easy.

How are you checking the spark? Are you using an open gap tester? I find that Bendix systems can reliably jump 3/16" and a really ideal one can sometimes make a 1/4". (Bendix ignition is kind of a joke. I may have had a bunch of weak magnetic rotors; but this has been my personal experience with many of them.) If you’re using inline, neon checkers, be advised that if the cylinder fouls out, like from water drops from a bad crank seal or over-abundance of fuel, the neons will go dim, as if they have no spark. This is because the voltage to fire a fouled plug is a lot less than to fire a normal one, like 2000 vs. 7000 and up.

People will gasp; but sometimes out of desperation, I have closed the plug gap to .015" and the result is a fine-running engine.