I had to replace the coil wire on my C9,not a big job,but you'll have to pull the whole ignition system apart to do it.(at least I thought that was the easiest way to make sure the wire was threaded in properly) I had to replace the coil as well.
I believe I have a stack muffler here with your name on it.
Rather than JB Weld it,see if you can get a helicoil to stay in a bit further down if there are any threads left at all.The threads go down a bit further than the screw does,so it might be possible to go with a longer screw.Don't drill down any deeper than the boss is already,otherwise you'll be into the combustion chamber.Measure the depth of the hole & compare it to the length of the screw.
I just remembered about "Blue Demon" brazing.This is where you'll need to use either oxy-acetylene or mapp gas for your heat source,propane won't get hot enough.The metal must be brought to 600*& you use the special brazing rod.The end result is stronger than the original metal,so I don't know if it can be drilled or tapped afterwards.Probably better to have the screw/bolt in place & fill in with the braze,then let cool & turn the screw out.The braze will not adhere to the screw/bolt as it's much harder.Just Google Blue Demon brazing & you should be able to find a video on YouTube about it.