Welcome to Stack Exchange! Unfortunately the parameters you need are not really relevant to the PTC data sheet. For a motor temperature sensor, the PTC thermal time constant is negligible compared to the motor and the thermal contact resistance is related to how the PTC is mounted. Resistance to ambient is a motor constant.
Here is how to go about it. Run the motor with a fixed torque load and a fixed speed until the windings reach equilibrium temperature. You can measure the winding average temperature by measuring the winding resistance at room temperature, then again when it reached equilibrium after running and using the temperature coefficient of copper. Remember that this is the average winding resistance and there may be some hot spots near the end windings. Anyway, you can now measure the power you applied in watts at equilibrium, so the thermal resistance to ambient is the amount of temperature rise (from ambient) in °C, divided by the power in watts.
Next measure the PTC. Your contact resistance is the difference between the PTC reading and the winding temperature, once again divided by the motor power, and should be a small number if you mounted the PTC well. This number must be calculated and added to the PTC reading to approximate the winding temperature at a constant load.
No shut off the motor. Make a plot of the winding temperature vs time using the winding resistance and calculate the time constant, then (when it is completely cool) power the motor up again under the same load and make a plot of the winding temperature as reported by the PTC (with the contact resistance term added) as a double check. The time constant is necessary because the windings will get hot in a stall but the thermal mass of the motor means that the PTC reading will lag, so you need to shut down early if the slope is high. Use whichever number is higher for safety.
