Ok, the coil is mounted on the frame. The electrical input to the coil comes from the stator to the coil. The coil grounds to the frame and back to the stator. The stator grounds to the engine. If the engine is not making good electrical contact through the frame or head stay, then you don't have a good ground on the electrical system and thus can have either a weak spark or no spark. The best way to insure this good ground is complete is for the engines head stay to be clean as it mounts to the engine and the frame. This is the same issue found with a lot of electrical issues in auto's when the ground straps to and from the body and engine are not clean and free from rust.
Remember you have to have a power source and a ground on any electrical system for it to function.
The engine head stay is the bracket that bolts to the head from the frame. It stabilizes the engine, but also is the major ground connection between the power source ( stator) and the coil and the ground.
FYI: I am not a big fan of auto adjusting multi meters for testing motorcycle electrical as the readings given don't always use the needed range to the testing so the results can be checked against the service manual.
Paw Paw