There are several ways to test for spark, and spark timing, as well. If you have an inductive timing light, you can get a relative idea of both, simultaneously by rotating the engine to where one of the pistons is very near top dead center in one of the cylinders, and make a reference mark on the PTO with a sharpie pen, then connect everything as normal with the timing light connected to the plug wire of the cylinder under test and crank as if starting. The timing light will sense the plug wire pulse and flash, the mark made on the PTO wheel should be illuminated close to where you found top dead center on that cylinder, ie: you should see your mark in the timing light flash just ahead of TDC.
If there's no flash on the timing light, it didn't sense a pulse on the plug wire, thus you have a lack of spark, or if the light flashes but your mark isn't in the flash (because it's around the other side near the bottom where it's impossible to see), then the timing is nowhere near top dead center (maybe 180 degrees out of time?).
That's where I'd start, b/c it's an easy fast test and I have a timing light (I bought this timing light with my teenage lawn mowing money several decades ago). You could just use an inline spark checker as well, to get an idea of the presence of ignition energy to accomplish parts of this, although, just b/c you have full ignition energy doesn't guarantee the plug will fire an air:fuel mixture if the plug is fouled. New plugs should eliminate that possibility.
All these engines use a plug gap of ~0.020", about the thickness of most credit cards. Excessive gap is more likely to produce misfiring at higher engine loads such as full throttle acceleration, where it's most challenging to fire a plug.
