Viton is good stuff
I used a sheet of Viton to make some gaskets for the new valve cover. The first one was just a washer that fits under the acorn not on top of the cover:
I had problems with the valve cover gasket leaking. At first I was using a cheap Chinese copy of a gasket and after a few attempts I gave up on it. I then used the gasket that came with the Lifan engine. I had no leaking problems with the stock cover, so I thought this would take care of it. But this one leaked as well.
So breaking on the Viton sheet again I made a new gasket. No leaking so far: