I've been running Falken Azenis 615's for a few years (well, too long actually, they are now hard as rocks after 6 years)... but they work well.
If running PCA, small region classes, you are likely in a large class of possible old cars.. Thats how it is for me.. 356 through 986 boxster and including Cayenne. We don't have many people come out for that class (at most 2-3), so its more about the experience for some.. but its nice to surprise those new cars that have more than 100hp
Any 205-50 15 "r" comp will work well enough. Lots of choices too, due to that being a "miata" tire size and well, miatas are everywhere. Find the one that fits your wallet and ego !
If you are running "improved", though, not sure what your class limits are, or what other types of cars can get grouped in with you. That said, as long as its not a HP course, a sticky enough tire can still be enough to mix it up well.
Years ago I ran an "improved" class 914/4. I was at the PCA ZONE 1 autocross - 2 day event. After day 1 on the "HP" course, I was in 6th place behind a string of 911s. Day 2 was the "drivers course" (ie, so tight and technical that HP didn't help) and was second that day by just a smidge.. Ended up 3rd overall. This was also back when "60 series performance tires" was not an oxymoron too.
Go, have fun.. and let others bask in the prowess of your "50 year old car"