Watercooled electric motors for this weightclass are normaly motors used in boats, and are actualy overvolted, or hot wound motors with a waterjacket that gets cold water from a scoop and exhausts the hot water at the back.

To use these motors in a feather, you'll need to add a small pump (can't rely on thermosyphon) and a not so small radiator.

Easier to use such motor and dump some dry ice around it. Then you just need a kind of chamber around the motors.