Mr. Enginenut
All of your points are very valid provided with the fact of having an engine that was working fine with a sudden or gradual decline in performance.
In this case it appears that the engine is not running correctly. Since none of us are sitting under the same shady tree as this particular engine and can only offer assistance based off the infomation given, all of the points posted before you are good ones.
I have gotten many old engines to run like a top and I approach all of them with the same path. First determine if the engine is getting fuel, spark, then see if its getting them when in needs (timing).
Your first point is often the reason for not starting a newly rebuilt engine. The timing is set, then the clap isn't tightened completely or at all and the engine slips out of time. When this happens, more often than not, it will backfire out of the carb.
