I bought American Standard and Korky replacement flush valve seals and neither stopped the problem on my one piece #2004 Champion.
As I was turning the cylinder that the ring mounts on I noticed that there are three "ears" on the cylinder and three nubs where they land in the seat area.
I used a standard plier to grab each ear and bend it up. I reached in through the slot where the float that has some unknown function mounts.
Now the seal ring is able the press down on the seat and it seems to work fine.
Best of luck,