Would it be possible to make a mod that replaces the class S cars with race cars in arcade mode and have the ai drive them?
In theory - the GT2+ change here to swap the Clio for the Esprit is a template on how you could do it. You'd need to copy over all of the parts data for each car from the GT mode data to the Arcade mode data, then patch the hardcoded car list in the Arcade overlay with the new car IDs, and update the hardcoded stats shown in the menu. All of the attract demos that use the Arcade data will also break, and will need to be removed or re-recorded.
Probably not recommended for your first mod though, as you may also need to figure out how the manufacturer logos work, and may run into unforeseen issues if it was never written to support race cars. It'd be breaking new ground either way.