In case you don't want to go through great lengths sourcing them I have Viton rings for 1100/11-87 for cheap, just send me a pm for more info.
Thanks!
I just got a 1100 made in 1979. It has never been fired and still has the original o-ring. I plan to shoot it with the original o-ring first to see if its actually still functional. I cleaned it up and put some oil on the o-ring to see if it will soak it up. It didnt feel brittle but definitely a little a dry.