16-02-17, 11:34 PM
Haven't come up against that problem myself but I know you can get extractors that are reverse thread and as you drill down into the plug it grips and unscrews. If you don't fancy doing it you should be able to find a bike shop that would have a go at extracting it. If it was me I'd probably take the head off. It's not as difficult as it sounds if you've got reasonable skills with tools and can follow a manual. The trickiest bit is getting the exhaust off without shearing a stud. You need a low end torque wrench and a new head gasket