I would sign up as your first member!
(And as such I would be willing to help you in whatever way you needed that I could.)
I've been looking at the online racing leagues awhile, and can't find a league that already races the lower performance end of antique road cars. And no one-make races in that.
For example, there's a Historic Race Car league (not sure where to find it right now) where they race the historic RACE cars. (the ones you'd be racing in the Aspec historic race car in the Expert series)
Making a
non one-make gets dicey because you have to extensively test the compatibility & competitiveness of each car with the others, at various restrictions (either hp/weight or PP#).
It gets pretty complicated.
The kei-car league
Low HP Championship I've participated in (which is really nice group of people btw & I highly recommend), allows antique road cars, but mechanically, obviously the newer cars have an advantage. Especially if they have more gears they can utilize.
Clearly even if say, *I* had a mechanical advantage, there are many people who could beat me in a disadvantaged car, so it really depends on driver skill too.
But you get where I'm going with this.
Ideally I would like an antique car league that does one-make races of the various antique road cars. Or something like that.
I've seen other people expressing a similar interest at times. But I've yet to see an antique road car specific league.
And when I say various antique road cars on the lower hp end... I'm talking I'd be interested in some fun one-make races with everything from the Mazda Carol to a road track style race of SuperBees or Mercury Cougars.

(ie: non drafting battle race of an antique muscle car)
You know with set tire restrictions & maybe PP restriction would be easier (less complicated) than restricting what types of mods would be allowed.
I'm an avid collector of
all SMALL cars in the game, and
all antiques. And my favourites are the SMALL ANTIQUES. heehee.
I don't care if they're "slow"... they're fun... at least to me.
There's plenty of opportunities to play with the higher-end cars... There's already dozens of leagues for that in various ways. Small antiques seem to be a niche interest of perhaps not a big group of people.
But I've been in online races with just 3 or 4 people, and had loads of fun. Even if I come in dead last, lapped. haha.
So even if your league only has an average of 4 people in each race, I think it'd be worth it.
Let me know if there's anything I can do to help!

And I'll pass the info on to people I already talk to on the forum that might be interested.
note: I just looked at my post & wanted to add that don't grin quite so much in real life. (that would be kind of odd. LOL)