Ian: I'm not the expert that Matt is, but my guess is that there was a valve sticking if there is a hesitation of the period you describe. That is, there are two valve openings for every four revolutions of the crank.
My other thought, about the weak spark, is that, depending upon the vintage of your motor, there is an air gap between the flywheel magnet and the magneto that must be adjusted, as well as possibly a set of contact points, if I recall correctly. The contact points were eliminated in the later versions, again, if I recall correctly, and a "solid state" ignition system was employed in the later editions of the motor. You have to pull the flywheel off to check all this, and the nut is torqued to something like 140 ft-lbs. Plus, the crank is tapered, so you need a good puller to get it off, a steering wheel puller may work (two bolts), but I would recommend a harmonic balance style (five holes; either three or two bolts), it is stronger. Before you go to that kind of trouble, though, I would try a new spark plug; and/or lay the spark plug on the engine and try to crank it just to see if you are getting a spark at the plug itself, not just the tester. The tester is fine, I've used one, but it does "get in the way" of the spark getting to the fuel.
That being said, if there is an issue with the valve not closing, and I suspect there is, then the engine will never develop the compression necessary for combustion, even with the strongest spark. And while the engine will appear to run fine on only one cylinder, you'll still have problems in the end. Matt is correct in that the only way to tell what is really going on is to pull the heads off --it ends a lot of guessing really quick.
However, if you're getting the 80-90 PSI compression on BOTH cylinders specified in the service manual, then I would say the mechanics are OK, and focus on getting spark to the combustion chamber. With combustion and spark, engine starter fluid, gasoline, brake cleaner, carb cleaner or any number of other volatile substances sprayed down the throat of the carburetor should allow the engine to run for a minute or so. Then you can delve into the fuel delivery issues.
Just my two cents.
The more cautious approach is to check all things mechanical before adding electricity and gasoline.