You'll notice that I did not recommend the thermostat as the problem. The engine ran at the correct operating temperature under normal loads. It's unlikely that the thermostat won't fully open at higher temperatures...........but, not impossible.

When the engine is very hot (above 100°C)..........shut it down and simultaneously observe the fan. If the fan stops with the engine, the clutch is fine. If the fan spins several revolutions after the engine stops, the clutch is toast.