Re: BOLT head in the Bilge...
Another couple of options: Oddly enough, an old basketball or soccer ball makes a decent air jack. Deflate, stick under the engine, pump up until it starts to lift.
Once you remove that engine mount, and put some PBlaster or other real penetrant on that stud, you may also be able to unscrew it by cutting a slot in it (with a metal saw blade or dremel bit) and then hitting it with an impact hammer and a "screwdriver" tip. If the penetrant has done its job, that will spin right out with the impact hammer. If not...you can still use the screw extractor.
Note the position and alignment of the motor mount carefully, try to match it back up since that affects shaft alignment. In theory, engine mounts should be replaced about every five years as the rubber stiffens up or cracks over time. In practice...that's something else again.
Could be that bolt sheared because it was improperly torqued, so if you have a torque wrench, use it. If you don't, this could be the time to buy one anyhow. And I suppose if you were really compulsive, you mght want to replace both bolts on both front mounts while you're in there. Using Loctite or antiseize on all of them as well.
Or just replace the one and go sailing.