Is there enough at the back to get a hefty vice-grip pliers on it?
The trick to getting those out is HEAT. Lots of heat . The HEAT and QUENCH techniques. Don’t even try without heat; if you do get the screw to move it might damage the threads.
In your situation…..if the heat and vice-grips is not the answer….then you have to drill it out. I might be looking for another clamp if it’s a common motor.
