Using PWM has a lot of advantages. First you don't step up the motor, it will increase rpm with pulse length. You can use PWM with an astable oscillator and varying pulse lengths.
Wire a 555 accordingly and hook up an N-Channel Power MosFet. The circuit is much cheaper than using a converter and you don't have the disadvantage of very limited torque when starting at low rpm.
This circuit can be done within 30 minutes. The frequency varies slightly with pulse width. Pot position is near 100%. Shortest pulse width 2.5microseconds, longest pulse = steady DC.
Boncuk