Are you leaving the base of Q1 open on purpose? If so, what do the two 1K resistors on the left do?
If you are trying to use the thermally induced leakage in the open-base Q1 as a temperature sensor, then this leakage is tiny compared to the current required to turn off Q2. If anything, the motor should be running all the time. If you are trying to use the reverse leakage as a temperature sensor, you would have to use an opamp or two to amplify the thermal variation to control the base of the power transistor Q2.
What is the purpose of the pot and resistors on the right?
Use a thermistor as a temperature sensor.