With the engine conversion parts places like Wacker Engines,Carr Precision and Jag Engines disappearing, you either have to have the equipment to make your own or get a machine shop to make parts. Only place left that I know of is Lamberts RC
LambertsRC A few guys on various forums make their own conversion parts and maybe would make yours.
I think there's a guy on Ebay that makes custom rings for engines,but can't remember what he goes by on Ebay. Frank Bowman used to make them but also quit doing them.