As you say this sort of thing is very frustrating and if you have completed a nut and bolt restoration it can be many things. If you have had it running then it would not seem to be anything major and I would not have thought it to be valve timing. I would check the points gap and ignition timing. I would check the spark by removing the plug and earth it to the engine and make sure you have a good fat spark. I would then turn my attention to the fuel side. Make sure that there is a good flow of fuel, check float level is correct and carb adjustments. Check there is no air leak between the carb and engine, this can be caused by overtighteneing as well as undertighteneing. Dont give up, you will find plenty of advise on this forum and just keep in mind when you ride off for the first time that sound will be a joy. all the best.