Your spark returns to the mag plate thru the top of the brass retainer ring which is also a support bearing for the plate and also thru the center bearing which the plate rotates on. These areas must be free of any corrosion or buildup of any kind which can impede the electrical flow. Lightly sanding the inside of the plates bearing surface and other contact surfaces will help to ensure a good electron flow. Don’t bend or deform the brass plate.
To determine if the plate is too loose, try wiggling one edge of it up and down while holding the opposite edge still. A good fitting plate will hardly move at all while a loose one will flop around quite noticeably.
If it isn’t poor grounding causing the spark issue, I’d then suspect internal breaks or shorts in the plug wires or poor connections on the ends of them.
