To add another possible source you might try oldroads.com. They have numerous discussion boards. Scroll to the Vintage Lightweight Board and post your info. Nishikis get mentioned quite abit and I believe that John E. (from this board and OldRoads as well) used to work in a shop that sold Nishikis during the timeframe of your bike.
Posting is easy as pie, don't even need to join.