Master Cylinder Sight Lens 18mm - (Pair of 2), Premium Set Replacement with O-Rings for Hydraulic Brake Lever is what I used for my 1985 Aspencade. Works far better than the plexi that I used last year for the master brake cylinder, and no sealant to dry.
Bought it from Amazon, for 9.00, a little harder to install but a far better result. I used a couple of thin pieces of wood and a very small (3" I think) c clamp to get it most of the way in, then used a hammer and another piece of wood to make sure it was in all the way.
With a new o-ring and push fit it held fluid immediately, and the sight glass has a space between the outer face and the sight metal disk so it's easier to see the level.
When replacing the leaking window be sure to remove the parts shown in the picture.