I,m stumped

FYI, this is not an OHV engine so it doesn't have rocker arms. However, this engine does utilize valve seats inserted into the block. I have seen them come loose and in some cases completely disintegrate when they come loose. To check these, you'll have to pull the head to examine the mating surface where each of the valves close. It's usually pretty obvious if there is a problem with a seat that has come loose. Before pulling the head, I'd do a compression check just to see where you are. A valve seat problem can manifest itself in a low compression reading. Same with a burnt exhaust valve causing a poor 'seal'.

Regarding the other poster's question, this engine does not utilize points and condenser. You should check the spark with a tester to verify it. It should obviously not be intermittent, especially since you put a new coil on it. For the $2, I'd put a new plug in as well. This is a pretty simple and reliable ignition system.

Keep us posted!

I would ask that you be a bit more specific now. Your initial post said intermittent spark. Then no spark. Which is it? IOW, does the first(original) coil produce intermittent spark, and the second(new) produce none?

Secondly, very specifically, how are you testing for spark? Tester? Plug against the head? Other? Have you replaced the plug with a brand new one(not another used one)? Can you snap a photo of the ignition coil on the engine and share with us? Other than the spark plug wire coming out of the coil, there are no other wires connected to the coil assembly, right?

Let us know!