Here's information on the 2nd annual Indy Legends even this weekend at IRP. Spectator admission is $20 and is good for both days. Cars run from 9:00-6:00 on Saturday and from 9:00-4:00 on Sunday. I don't know the details but I have heard that the IMS Museum is bringing some of their cars. Last year they brought several cars and ran them on the track both days.
https://indylegends.com/