I needed a that gasket once when I changed my sprocket and NFG/CMW sent a tube of Permatex. Take it from there. I don't order gaskets any more.
To tell if you're chain is worn out, grab a link that's on the rear sprocket and pull up on it. If it comes up a half tooth, it's worn out. If you have done a few adjustments, it's worn out. If it needs an adjustment every two or three hundred miles, it's shot. Those OEM chains don't last long. Because it's an Oring chain does not guarantee quality. Remember, RE get their chains from the lowest bidder, as do all motorcycle manufacturers. Most OEM chains are not very good.
Bare