What are you trying to improve on? longer stem? larger mirror?
I bought mirrors for my 85ZN700 because the stock mirrors stems were too short and I coundn't see past my shoulders. I found some long stem mirrors at J&P Cycle that even had "Kawasaki" on the mirror face. I had to know the thread size to order ones that work. Maybe no aftermarket mirrors are made for you specific bike and you will have to figure out what will work?
1985 Kawasaki ZN700 Shaft, 4cyl
1989 Yamaha Venture Royale