When I acquire a used engine the first thing I do is take it apart and inspect everything then clean it. It is sounding like you have crud in the low end of the carb. If it was gunked up and not working when you got it then that would be a good clue. If nothing else, remove the carb and soak it in alcohol or carb cleaner for a while, take it apart and clean it some more. What happens if you flip the plane upside down and remove the igniter?? Does it do the same thing or does it keep running? It could be a fuel tank location problem but it sounds like hardened Castor in the carb. Keeping heat on the plug isn't an answer, it's a band aid. I have run a lot of inverted .91s without a problem, the engine doesn't know it's inverted. With a used engine I use a starting stand and run and tune the engine while it's on the stand and not in the plane. My stand has a Velcro strip that holds the fuel tank so I can set it up correctly with the engine. If it runs well on the stand and not in the plane then I start looking at the tank location. From your list everything sounds OK. For engine cleaning I have a sonic cleaner, something I found that my wife used for cleaning her rings and what not. She hasn't missed it yet and it's what I use to clean my stripped down carbs and parts. Compressed air is also a big help. You know it's in the low end. If someone has taken the engine apart the cam may be out of time?? Take a look at the cam gear and make sure it is in correctly. I'm just shooting in the dark not having the engine in hand so these are just ideas I would have done. It's something simple but it's up to you to locate the problem.