Farmer's use a "quality" penetrating oil.....I use Kroil.....and let it soak for a few days and then get out the heat.....like a Bernzomatic utility torch in your case.....whereby we use Acetylene/oxygen torches since we are usually after large fasteners with decades of rust build up.
Obviously you need to wipe off the oil and protect the areas that the heat will damage. Recalling some Johnson-Evinrude engines, the connection is in the middle of the mid section and the only thing you will hurt is paint where the flame hits the aluminum surface on the other side. You might take a soda or beer can and cut out a shield to put behind the coupler when applying heat.
Then a couple of long nose pliers (required to fit in the slot) used in a twisting, back and forth motion, with pressure in the disconnect direction as you get the coupler moving back and forth. Good Luck!