Check out any of the nineteen Cowling components enlisted.
Parts such as the Cowling: Rivet (5m0), Cover, Upper, Cover, Lower, Plug (341-26329-00) are included in the Ca50l 1984 Riva Cowling parts fiche.
The most reasonably priced component used on the Ca50l 1984 Riva Cowling is the Ybs66-5 Washer, Plain.