I would pull the flywheel cover and make sure there is an air gap between the mag and the flywheel. It should be a small gap, I gorgot how many thousandths it should be, but something like two shhets of paper.
Make sure everything else looks good inside that area, nothing weird going on.
Next, turn the flywheel by hand in both directions, check to see if the clutch bell is turning, if it is, it could be the pinion screw has come loose and the clutch shoes are making contact with the inner face of the clutch bell.
Keep the spark plug out during this testing. Spark plug stays out until you determine what exactly is causing the issue.
Mike