Sounds to me the coil or the plug is failing when hot. Put the correct plug in it and see if it works like it should. If it fails again immediately check for no spark while it is hot. If no spark, the coil is suspect.
On a side note, make sure you oil is clean and full.
Last but not least is possible debris in the fuel tank. The tank is hidden rather well on these, and there is no telling what has collected over time by refueling with grass clipping all around or possible prior owner had a dirty fuel can. Debris floats around and will starve the engine for fuel.