Others who might respond have far more expertise in this area than I but here's a couple of thoughts.
A. There is likely to be some inductive spike from the motor that might damage a transistor. A diode or diodes, appropriately connected, can help to solve the problem.
B. A transistor that will handle the required current ( I did not look at the motor) may require more current from the pot than it can supply.
C. An op amp configured as a comparator - or a comparator might be a good way to accomplish the triggering - taking the load off the pot and it will be on or off - not between. The output might drive a transistor so that it's full on or off - easing the demands on the transistor heat sink. A small transistor might operate a relay that has the current handling needed. The relay can also produce inductive spikes so protection is still required.
I have some old Radio Shack books that have lots of circuits for driving small DC motors and I am sure that such simple circuits are published elsewhere. The hard work is done for you so you should not have to struggle.
Hope this gives you a start.