Keith,
It's a 1988 Mercury I6, 115HP. I tried an impact driver and that's what broke the screw initially. I've tried an easy-out. I've tried rapping it. No dice so far. If I thought it was only being held in by pressure, I would just drill through it, release the pressure, and put in a new one. But I'm a little afraid it is seized up in there. I thought removing the trim motor would release the pressure but the screw still won't turn. My two piston rods are still fully retracted, so something is still under pressure.
I haven't found anyone in my area willing to work on the motor. Too old.