What was the ohms reading on the secondary winding from ground to spark plug terminal? please don’t measure at the plug wire end but at thecoil terminal. The coil or capacitor are usually the culprets. Did you ground the mag plate to the block or check with the ohm meter for hi resistance while advancing & retarding the throttle. Use an analog meter to watch the indicator needle jump up & down 4 best results.
Have you checked the clearance between the coil laminations & the flywheel magnets? Put a short piece of masking tape on the lams & rotate flywheel by hand & remove flywhl. Get as close as possable without rubbing on the tape for max energy.
P.S. Had a Merc mag so hot it turned the points gold from heat. It was a current reverse from one coil to the other, 2 cylinder mtr. I gounnded the mag & no more problems, try it once!

P.S. For everyone building olg GM cars, the 6 volt starter fields were insulated with PAPER! Too many rain storms & the paper goes away.
Super corona Dope fixes that, Same for outboard electric starters.