Tractor will run, suddenly stop, after sitting start again

Conventional (non fuel injected) engines may be easy to diagnose by checking for three things; air, fuel and spark. Assuming the engine was maintained with periodic oil and air filter changes, air is assumed easily ingested. This leaves fuel and spark for testing when the engine dies. Fuel is verified when starting fails and removing spark plugs; fuel should smell on plugs. Spark should be tested with plugs removed, reconnected to wiring with plug bases grounded to the engine while starting; without compression the engine should spin faster and spark should be seen on plugs. In your case of the engine dying after running awhile, intermittent loss of fuel or spark needs to be tested at the moment the engine dies. I lean more towards an intermittent ignition system.