PSA: Old champion's kits CANNOT just be brought back!
I know that it's a really exciting idea to think of - A gamemode with all of the pre-rework champions, I certainly would love this just as much as the next guy.
Unfortunately, Riot has stated (I can't find the source D:) that they get rid of old champions' kits once they're reworked, so there's no way to bring them back.
You may not trust me in saying this because I'm not providing a source, but think about it, everyone and their mother has had this idea, if Riot could do it, why the hell wouldn't they?
And because of this, I also think people should stop pretending they reinvented the wheel by thinking of this gamemode. I'd say 80% of the community probably has.
Theoretically, they could just code the old champions fresh, but the amount of time and effort that would take is not viable in the slightest, it would be like making dozens of new champions just without having to come up with the ideas. It's not feasible.