the Duster choppers used black Tecumseh H35's those bikes were roughly 1970-1975 so they were the typical mini bke era curved top shroud with plastic side fill tank:
Heres a full on shot of that Duster I had back together, as you can see your is basically only the frame of one anymore, those nanza forks and bars could be a good bargaining chip though if you wanted to go back to chopper.