Yes, the original gasket is a fiber material, mine was toast, badly distorted and likely wouldn't seal worth beans. The different Mustang pasts vendors sell a rubber replacement gasket but I decided to make my own using 1/16" rubberized cork. I was also concerned it would tear but it held up better than expected. Had to stretch it a fair amount to get it over the nozzle but no problem, slipped into place easily. We'll see how it holds up, if there are any issues I'll go with one of the rubber replacements.
Rivets are drive screws, total length 0.40", shaft diameter a little over 1/8", I assume to drive into a 1/8" hole. Cobra valve covers use the same thing for the wire loom baffles, I think those are slightly larger?