The O-ring comes in the carb rebuild kit, which is how I got a new one. Even with my new O-ring, I still top up my oil about once a month. On my old Midget 1500, I used to top its damper oil up about twice a year. Victoria British has the O-ring as a separate part; that's where I got my rebuild kit, new metering needle, and my adjusting screw.
Scott