The one big issue would be that they need to find one in good condition to model from which may be very hard to gain access to.
Kaz is one of the judges of the Pebble Beach Concours each year, nevermind that most big car brands they have good connections with already, have their own museums.
Then there's a whole scene of private owners of which a lot would no doubt be more than willing to offer theirs to model if asked.
I doubt the issue is gaining access to a few iconic 20's or 30's cars (I mean, they modelled the sole existing XJ13), nor finding ones that are in good condition (in fact, some of these cars are harder to find in a bad condition as they're mostly lovingly cared for over decades, especially the really exotic ones).
The issue is whether Kaz really wants them included or not, if he wants them he has all the means and connections to find them.
True, but there's only one KTM (and it's ugly I might add). You won't likely see a field of Vintage cars was kind of what I was getting at, but it would be nice to see one or two, especially in conjunction with a livery editor!
Don't get me wrong, I'm not asking for an entire field, just a few (even just one right now) would already be enough, although this era deserves more respect than just a few it has to be said.
Who knows, maybe due to this being a largely unknown (and therefor unpopular) part of car history for a large part of the demographic that currently plays videogames, adding some will have the scales falling from their eyes and they'll demand more.
As for a livery editor, of all the cars you could use as an example to use a livery editor for, this has to be the least suitable.
I'd love a livery editor but I doubt I'd use it for these cars....unless....you can also use it to make them look rusty and scruffy like taking a barnfind directly to the track.