You have an 'internal' resistance of 31.25 ohms that is the fan winding. If you put a 100 ohm resistor in series with the fan, you have 131.25 ohms of resistance. That will allow 38 milliamps in your circuit. The 5 volts will be split with 3.8 volts across the resistor and 1.2 volts across the motor winding. Approximately .19 watts. Now whether that fan will start/spin with that resistor in series is a guess. Try a 33 ohm resistor as a second choice and see if that allows the motor to run.

Remember, the 5 volts will remain at 5 volts, the motor's internal resistance will remain constant. You are varying the current by adding resistance (and therefore lowering the power). Less power = slower speed.