Replacing a 1.5V with a 5V power source
Just adding a resistor is not the right way to go.  The current draw of a simple DC motor changes depending on the load. In an unloaded condition the current will be at a minimum. In a stalled condition it will reach a maximun.  To run a 1.5V motor from a 5V source you should step down the voltage with a regulator IC or a string of 4 or 5 silicon diodes, (the diodes will need to be rated for at least the maximum motor current, or best guess in this case). Using a USB from a PC might be risky if you do not know the motor's current draw, best to test out the idea using a USB wall adapter first.