I'm no EB, but having dealt with this type of situation in my Yamaha OB motor that was used in Salt water before I got it, I understand the picture.

If you broke the plug off, there is probably some type of corrosion involved between the dissimilar metals and gunk has got the threads all locked up. The solution that an old hand used to overcome situations like this was to tack weld a bolt head to the remaining material to be removed and then use A LOT of heat from a Oxy-Acetylene torch to heat the Aluminum while twisting on the bolt head with the proper wrench.

I, myself, didn't bother to do this repair as the casing bolt that snapped off wasn't imperative to the integrity of the casing. Additionally, I had snapped off a hardened easy out and couldn't remove it, so the reward wouldn't outweigh the work/price of the repair <img src="/forums/images/graemlins/shame.gif" alt="" /> <img src="/forums/images/graemlins/rolleyes.gif" alt="" />.

I don't know how much of my rambling will be helpful, but please don't be too aggressive with that easy out because it WILL make your life more complicated if you break it like me. Good luck!