He wants to get nearly 12V across the motor when the pot is at one end. With an NPN emitter follower, he will get no more than 4.3V. The motor will never start.
Painstream, with the circuit you have (changing the NPN to a Darlington, or not) will give you very nonlinear response to the pot. I showed you the plot of voltage vs pot position for the PNP circuit. The minimum motor voltage is (12-5.7)=6.3V. The maximum is 11.3V.
I considered suggesting PWM, but I guessed that you didn't want something so complex.