Re: impeller shopping
The thermostat cover is held in place with 4 large screws. Sometimes that can get corroded and pretty stubborn to get out. No need to take out the head bolts or remove the head cover.
To test the over-heat sensor itself, you can remove it from the head and heat it up with a heat gun, or maybe a blow drier. I'm guessing that the sensor is set to short at about 180?. Check it with a meter between the wire and sensor case. When it's cold it should read open. When it's heated up, it should read shorted.
To test the over heat buzzer, turn you ignition key to the "ON" position and ground out the wire on the terminal block that the sensor goes to. The buzzer should come on.