It;s been a while since I had the pleasure of treying to persuade one of thse to start, but two thoughts for you:
They are very sensitive to exhaust back pressure. I remember onre that would run happily on the back of a chair with the prop only just under the water, then when carried (running!
) and dropped onto the back of a mirror dingy it coughed & died.
Also have you absolutely flooded the carb 'till fuel is dripping out? I've known two that needed that for a cold start.......