Thanks for the replies, I've got a screw in muffler here but would really like to get it back to it's original 'spec'. I'm unsure why the bolt failed as it was like this when I got it, but I can tell you it has an angled edge all around and almost looks like it's already been center punched (see below).

I've been hitting it with some penetrene every few hours and cleaned the area up a little since the last pic.

[Linked Image from dl.dropbox.com]
I know it's an ordinary shot but it's the best I've got..

To me it looks like it's been twisted off...

I've been tossing up if I should do this myself or just pay the $ to have a pro do it.. I don't want to get it wrong as there is not much room for repair and I'd really like to keep the original engine.