Replacing a thermal cut-off with a potentiometer
Not wise.  

This thing is designed to operate at the 350 ° and the design was tested to be safe under those conditions. It's definitely not made for higher temperatures, and when you operate it at these higher temperatures anything can happen. The thermal switch isn't just functional, it's a safety component. Don't do it.  

A potentiometer won't be enough, either. You would have to measure temperature (which is now done by the switch, which just recognizes one particular temperature), and compare the voltage the sensor gives with the voltage you set with the potmeter. And then you use the result of that comparison to switch the heater on or off.