Morning Fellas
Ok, I will set one thing straight, The two tubes in a pulsa jet operate very differently to what you would think.
So the small bowl and pickup is not a priming tube. The problem is there wasn't enough vacuum through the venturi of the carburettor to pull fuel from the bottom of the tank to run properly. So what these engines do is; the pulse operated diaphragm pump on the side picks up from the bottom of the tank and dumps in the top little bowl; all it ever does. The engine runs by pulling fuel from the little bowl into the carburettor via the low pressure created by the venturi just as in any other float type carb. This principle has stayed in use right up to modern times on all small side valve briggs engines.
This is why these engines can be a pain to start if they have been sitting for a while, or if the diaphragm is getting hard, if fuel does not get put into the small cup at the top the engine will never run.
As for you're problem, It shouldnt be too difficult to find a replacement tank. Any 4-5hp from the 70-80's will have the right tank, its just a matter of finding one. Keep an eye out on marketplace or gumtree. Alternatively try and find an older mower shop. I would be fairly certain that the ones out here would have a tank or 2 lying around
