It was not easy to find information on such events here; I do not know why but there was not much available on the web. At the end I heard about these races at an even organized by one of the vintage car clubs here where a guy told me about SOLO events.
The general website is
https://www.scca.com/pages/i-want-to-autocross (each regional club has their own site).
Once I got there at the race I discovered that there are other series going on here in St. Louis and I plan to check them out too.
It is possible that I could not find much on the web for these kinds of events was because I was searching for anything related to vintage cars. The SOLO races are not for such cars; they are mostly for guys that modify/upgrade their stock cars to the point that it is hard to tell where they have started from. Important to note that everyone was very friendly and helpful.
It is fun regardless of the car or the experience. I highly recommended as long as one is ready to accpet that the Fiat Spider is not going to be even close to the really fast cars.
I went there with my 16 1/2 year old daughter who drove my other car and beat my best lap time by 7 seconds. She claims that it was due to her superior driving skills (not because of the 335ix )....