I love to be able to take my car online and race them as they are fresh from being from the dealer with no modifications at all. Is there a way to do this with cars you already own that have every single part installed? I would hate having to buy a separate car just for this.
If you installed permanent parts such as weight reduction and engine tuning, then there is no way to revert your car back to stock. You have no choice but to buy a new one.
But if you didn't, just switch to a new tuning sheet for that car.