From the way I understand it you've eliminated the glow plug as the culprit and the fuel seems to be fine if your other engine still runs great on it. I'll have to ask this because it may be possible but have you pulled the head to check for any peeling? If the engine starts easily (hand starting is the best way to check it out), draws fuel easily for priming and holds compression well at TDC then basically the piston/liner should be in good shape. If all of this seems to be OK then my first suggestion would be to swap the carb over from your other engine and give it a try.