cl450nut thanks for the advise. I plan on tearing the engine down as soon as I'm able to get it out of the frame. The seat lock is gummed up (I've got it shot full of PB Blaster) so there really isn't much for me to do at the moment except make sure everything has a good dose of penitrating oil.
I spent a few minutes messing with the remains of the plug this morning. It looks like I'm going to need to find some kind of an extention for my extractor. Once in place it only extends as high is the top engine fin. I tried using an 11/32 Craftsman combination wrench on it but the open end is not strong enough and was deflecting. None of my other wrenches will fit between the fins and still allow horizontal movement to turn them. I used to have a couple 8-point craftsman sockets for situations like this (the top of the extractor is square), but I left them in a tool kit that went with my boat when I sold it some time ago. As cl450nut said, it may be easier to get it out once the head is off.
Below is a photo of the thing: