I would approach it like this:
Step 1: check spark directly from plug lead to ground (no spark plug or test box). If that is weak too, proceed to the next step.
Step 2: check the ignition module's ground connection. Does it ground directly to the crankcase? If not, there may be a problem in the indirect grounding path. Are both module and crankcase clean where the module attaches?
Step 3: disconnect the kill wire at the module. You've said you've already done that.
Step 4: check the flywheel magnet's magnetic strength with a piece of mild steel. I don't think it's likely to lose magnetism under normal conditions, but the magnet could be broken or have moved out of position.
