Let's say your motor is 3V and with 300mA current rating under full loading. Then you can expect no load current about 50mA (from my experience).
Roughly, for simple calculation, the no load impedance of a motor is about 60ohm (3V/50mA)(from ohms law).
To decrease current to half to see speed decrease you need to add another 60 ohms in series (your rheostat) (use 47ohm/1W).
When the rheostat is turned to 47ohms the motor voltage will be also halved (1.5v).
Now, the red led parameters are 1.8v/10mA.
You want the Led to be at full brightness at 3v so calculate the Led current limiting resistor R_lim = (3-1.8)/10mA = 120ohm (use 100ohms).
simulate this circuit – Schematic created using CircuitLab
As you are turning the rheostat you should see the motor speed changes and also the the change of led brightness.