Hi David, I am quite late with this reply and I rather assume you may have gotten things worked out by now.
For the off sized plug I would have shaved the plug to fit the socket. Jin's plugs are of inconsistent finish and do not take to wrenches and sockets as nicely as a standard 1/4-32 spark plug from nearly any hobby store or ebay. Just about any other plug from any other supplier will fit perfectly in a 5/16 or 8 mm tool.
Did you get after shaving the piston top?