The flanged OEM sprockets are discontinued and not available NOS anywhere I know of (I got the last one from Honda Canada about 5 years ago). No aftermarket sprocket I've seen has that sheet metal flange ring.
So unless you are trying for concours, forget the ring: it may have some mysterious vital purpose but I've used open sprockets with no problems at all. There are many suppliers that make excellent sprockets that fit, in assorted tooth counts if you want to adjust the final drive ratio.
Be very careful about buying used sprockets, a worn one will ruin a new chain quickly. Get a new set of high quality sprockets and a high strength chain and you will likely never have to replace them again.