Depending upon the current drawn by the motor.
You can use a LM 317 with a pot meter on the adjustment leg so the potmeter only carries a few mA's and the IC controls the current ( this is ok, up to 1 - 1.5 Amps).
Or, use a power transistor and control the base bias by means of a pot meter.
A 2N3055 can control up to 15 Amperes provided it is adequately heat sinked.
Better is to source a geared motor from a surplus shop and use that to drive your lighthouse lamp.