You don't want the motor to be gradually turned off and on over some range of light level. That would be bad for the motor, probably no good for whatever the motor is driving, and cause a lot of dissipation in the transistor switching the motor.
You need hyestersis, which provides "snap action". It will also make the turn off point a little darker than the turn on point. That keeps the system from jerking around the motor when the light is right at the threshold, which will have some inevitable noise on it.
Here is a circuit that should work:
In this example, I arbitrarily picked 20 kΩ for the LDR value at the darkness threshold you want the motor to switch on/off. Adjust R2 a bit to move the threshold.
Q2 and Q1 just amplify the lightness signal about the Q2 B-E threshold. Each of them inverts, so the output of Q1 will be the same polarity as the lightness signal on the base of Q2. R5 passes a little of this signal from output to input, which is how this circuit has hysteresis. The resulting 0-12 V digital signal is applied to the gate of Q3, which then switches the motor either solidly on or solidly off.