Might try a could of these tricks:
Keep a good spark plug in your pocket, as soon as the tractor stops running, pull one spark plug wire and plug the wire onto your plug,
now with spark plug pliers hold this plug against the motor block and turn over the engine.
Spark: yes or no,
if no then you have an electrical problem. I would change out the coil, that happened to me once, New coil solved the problem.
If you have spark, then you probably have a fuel problem.
When motor dies, before you hook up you spare plug, open the gas cap and listen for a whoosh sound, that would be air rushing into the tank. This can be caused by a stopped up fuel tank vent cap.
Your tank cap must be vented!!!!!
Also you can open the drain under the carb and see if fuel drains out, no fuel then vacuum on tank. You must have free flow of fuel.
This gives you two things to check.
May have to pull fuel cutoff and drain, wash out furl tank, add an metal inline fuel filter.
Also be sure your air filter tower and pan are clean.
So many things can cause this in old tractors.
Let us know what you find, others know so much more than me in troubleshooting problems like this.