I would start out with something like PB Blaster and taps from a small hammer. Let it soak and repeat if necessary. I would not use heat on it, but you can buy spray cans that freeze or lower the metal temperature which may help create some movement. Patience will help.
If the motor's been in salt water and the prop on for a long time, you may have to destroy it to get it off. Otherwise, it's patience and brute force, although some dealers have a pulling tool for stubborn props.
