As Bitsobrits says, the outer retaining ring on the rear sprocket is really not required. Early M3W's never had them fitted from new and some owners have removed them from later cars to reduce belt noises from slightly out belt alignment. If you can get one or have one made, no problem but I wouldn't let it stop you using the car. About 3 mm overhang on the rear sprocket is what the early M3W's usually had, although they also had the silver hub rather than the later black one your car will have.
