Thanks for the suggestions, all. I wasted another sunny day on this motor. Now it will barely run at all, and only briefly, and at about half throttle or above. I checked all the wiring on the switch box, found that the insulation had melted off the red wire, and bare wire was contacting the adjacent ring lug and nut, with some burn/melt marks around it. I'm not sure what that goes to but I put new insulation on all the wires which had crumbly or spongy insulation, and checked all the connections. I bought a spark tester, and installed it on the top plug. When cranking, it shows no spark. I thought it was defective, and exchanged it for another, which shows the same result. I did get the motor running briefly at half throttle with the spark tester on there, and I could see some yellow-orange sparks. This makes me think I may have weak spark, as the tester should show spark when cranking, and the spark should be blue-white. I pulled all the plugs and did the "poor man's compression test" by putting my finger over each spark plug hole and manually spinning the flywheel. I felt very solid, even pressure from each hole. Last time I did a real compression test, they were about even at 120. I don't think compression is my problem. Plugs all looked the same, and were slightly moist and smelled like gasoline. Tomorrow I will check the coils to see if they are within spec, and try to isolate and test the plug wires as well. I inspected the engine all over looking at the gaskets - all are intact, and all screws are tight. Fuel pump is new. Fuel primer solenoid works fine, and carb bowls seem to be full of fuel. The bolts for the timing mechanism are all tight, with no sign of movement since I set it to spec a few months ago. Weak spark could explain my problem, but it seems unlikely that all the coils or wires would go bad at the same time. The motor ran like a champ a few months ago. Maybe the stator/magnets/trigger may have an issue. There are a few tests I can run without pulling the flywheel that I will try if the coils are still to spec. I will try the staring fluid on the intake gaskets test next time I get it running. I'll let you know what I find.
Coils all meet spec. The primer solenoid works as intended. Still no spark. Trigger tests to spec. The stator is somewhat out of spec. Blue wire to blue/white measures 2200 where it should be 3250-3650. Hard to know if that is enough to cause no spark. I don't have a DVA tester to measure output voltage. Next I need to get or borrow one. I also don't have the type of gear puller to remove the flywheel in case I need to get at the stator, but I can borrow/rent one easily. It seems like my next move should be testing output voltages to pinpoint where the failed part is.