this ^^^^I presume you took care to set the clearance to the coil carefully.
Unfortunately, I can't think of much else there is to this simple electrical system. You've disconnected the grounding wires, replaced the coil and spark plug and the symptoms haven't changed. The only other thing is the magnet which doesn't seem at all likely. I'd start looking at other possible explanations.
Have you checked to see if the flywheel key is broken? Compression good? Fuel actually getting into the engine?
It might help to describe in detail the behavior of the engine when you try to start it, and any changes in it you have noticed when trying various repairs.
loosen the screws on the coils then put a business card between it and the flywheel then let the magnet suck it tight and then retighten the screws. a business card gets you the right gap but im not sure what the feeler gauge thickness would be if you rather do it that way.
as mentioned, the spark on newer coils isnt as big and bright but if you have a spark, even a weak one, it should start and run
i dont think the weak spark is your problem, hit it with a shot of starting fluid and if it starts then you issue is fuel supply to, or the carburetor itself so confirm the fuel pump is pumping as it should by removing the plugs and running the hose to the carb into a jug and crank the engine over. removing the plugs lets it get a decent rpm going for the test