Get yourself a magnetized allen wrench, I think that is the tool required for that plug, and carefully try to back it out. If you can not find a magnetized tool, get a 6 volt lantern battery and about 10 feet of 20 gauge enamaled insulated copper wire and wrap it tightly around the tool for about 1 to 2 inches, over lap the windings but you do not want any less than 1 inch or more than 2 inches coiled around the tool.
I'm assuming the tool is steel, not aluminum, and strip about an inch of enamal off the ends.
When you connect it to the - and + terminals of the battery, the tool will be magnetized. It's been a while since I've had to do this but I think the length and gauge of the wire will be about 5 to 10 Ohms so your battery should last about 30 minutes. If the coiled wire gets to hot to hold on to, you'll have to add more wire but first, check the tool as this process will magnetize it. To de-magnetize the tool, strike it sharply on a solid steel item.
If none of this works, I think you're going to have to remove the rear cover and reach in and some how apply pressure to the inside of the plug and try to force it back while turning it with the tool.
When you have sucessfully removed the plug, you're probably going to find out the housing is stripped, not the plug. Now you're going to have to re-tap the hole to the next size and get a matching plug to fit it.
I sure hope this helps you, been there, done that.