A shot in the dark but could it be something in the stator breaking down under load or a failing pickup sensor that isn't quite gone but is being very intermittent when under load?
Perhaps a mag has lost its "oomph" inside the flywheel so the sensor is confused and not firing properly when under speed thus not advancing itself which in turn causes excess bog and blowback and you get flooding which kills the cycle.
As others have suggested the crank or the only thing on the list would be the Flywheel, easier to try before you totally strip it down to remove crank.
All i can think of now is regulator, ht lead, suppressor cap or checking the power of the feed to the plug to make sure what looks like a strong spark, actually is!!
After that, I'm stumped. You appear to have changed everything else.