The sight window on MC was broken, I found replacement on ebay. According to the seller, the window is 18mm and the opening on the MC I measured at 18mm, too. However, the ebay listing said it is not compatible with Honda CB900C.
The price was two dollars and change shipped, so I took my chances. I cleaned and lubricated the opening with brake fluid, installed the o-ring and gently pressed the window in. It worked, and it does not leak now.