The two parts you broke are a common problem. Many dealers will stock them, but if not, the factory will have plenty in stock. I'd be inclined to get a machine tap that's just slight larger than the hole in the nylon (nipple) pc which is still inside the exhaust cover. (not one that's the size of the machine threads-smaller) If you turn the machine tap counterclockwise, the threads may grab the nylon pc and turn it out. Same as using the square end of a rat-tail file-inserted in the hole. The 4 rough edges of the file may be enough to grab the nipple end. You may be able to unscrew/extract it. You also may be able to force a small knife blade inside the hole. Cut the nylon pc with a sawing action. If you can cut it, it will loosen inside the hole. If you still can't get it out, last resort would be to remove the outer exhaust cover and drill it out and re-tap the original threads. I would not be so concerned if a small pc of plastic chip wound up inside the block. That's a water passage and any small nylon pcs will probably just pass thru the cooling system.
