Can you tell me what kind of hall sensor you use?- After a minute running the card was not too hot (around 38C degrees) however the distributor was at 68C (is the hall sensor sensitive to heat?).
Instead of plastic spacer I would make brass or aluminum spacer with cooling water holes drilled in it. Connect cooling water to spacer then run engine to see if that fixes the problem. If cooling does not solve the problem then you know carburetor is not getting too hot so spacer can be removed. Put a meter or tester on ignition system to see if is stops working when engine stops. I have new model airplane engines that lock up after running 1 minute but after engine stops I can spin propeller by hand. Apparently it only takes a very small temperature change for 400 degrees to 410 to lock up the model airplane engine. Factory model airplane fuel is 18% oil not enough oil for a new engine so I add oil to get 30% then engine runs good for 6 months until it is broke in then I use 25% oil in fuel. I assume your engine is 4 cycle not 2 cycle you can do a quick easy experiment run your engine on 2 cycle fuel to see if that prevents engine from stopping. Soon as you learn the problem then you can work on fixing it. If you can add cooling ports to the head that will help if engine is getting too hot. Car engine heads are the hottest part of the engine that is why engine cooling system thermostat never turns off cooling to the heads. Your engine look very nice & sounds good too I hope you can solve the problem.I'm with John. I think you might be having trouble with heat. Try making a 1/4 inch thick spacer from some type of plastic if you have some on hand. Had the same trouble on the V8 and the spacer worked very well
Check your cooling system edi. You should be able to run 4 or 5 minutes from dead cold. Maybe water flow has been restricted.
Water flowing to fast is also a problem. Is the pump as drawn or did you modify it.
Coolant is also a problem. Do not add coolant to your system. Try waterwetter.
Antifreeze adds surface tension that can hinder the absorption of heat. The water wetter breaks the surface tension and can aid cooling. It also contains the anti corrosion and lubricants without the glycol