Here is what I would do. First, remove the shroud and plug again and check for spark. Do this by spinning the flywheel by hand while holding the plug against the block. Then disconnect the kill wire from the magneto and repeat the test. If the spark is better with the kill wire off, you have a short in the kill wire, which you are going to have to find. If you have good spark both ways, the next thing I would check is the flywheel key. Post what you find.