Before this change is done it needs quite a lot of thought so it can be done once and well.
Firstly, what kind of cars are they? There should be a description in LFS that says what is stock and what is changed and how. Is the engine completely stock? Is the transmission "shorter" for better track performance or stock. What kind of tires and suspension parts. What parts can be changed or adjusted? And why?
2nd. What kind of racing are they are the cars supposed to be doing? If it is endurance racing, get rid of 2 litre 500hp engines. If it is fwd based touring car championship you wouldn't see UFR and XFR in the same grid. The UFR lacks credibility in every way and is in desperate need of rethinking. If the series is a small local small races championships, very limited rules would be used to keep the costs down. This should be the case in LFS as well. To replicate a series with low costs the low cost must be the primary goal in LFS as well. Just because LFS is fictional and non-limited doesn't mean that every option should be available just because there is no money limit.
These two are closely related and need to be decided before you can continue with any modifications to current cars. And obviously it would be good to have mechanics and physics updated for the limits to make sense and be usable.
One small part is the appearance of the cars. Atm the road cars are clearly road cars and look like unmodified too. But under the bonnet they are beasts built for racing. If the cars are road legal cars they would be used mostly for anything else than racing; they should look, feel and sound like standard versions. If the cars are really built for racing they should come with fire extinguishers in the cockpit, rollcages and race seats. Even trackdays have some limitations and even in amateurs race series the cars need to have certain safety devices. On the other hand standard does not mean that you can't change or adjust anything. What's the use to have standard road legal cars when you could make it a lot better with cheap tweaks, just like in real life?
And then finally comes the set up of the car. A standard road car like XRT, for example, would probably be understeery and a bit boring without any modifications. How much modifications is allowed and how are the cars supposed to drive?
Below are the LFS car classes listed with some comments regarding the aspects I wrote above:
UF1:
What year is it made and what year is now? Is it standard or modified and is it modified today or is it the Jim Clark special from the 60s? Or is it a mix of eras, like in GTL fashion?
XFG/XRG:
Defenately not hard tuned nor modified cars class. Maybe a series that has only two car models that are allowed to enter with tight rules limits on what you can do for the cars. Still, a road cage and racing seats in the cars. Other than that, stock engine and drivetrain. Possibly allow better shocks and springs to be installed (=some adjustability) but no suspensions mods that change the geometry for example. I guess the cars are allowed to be equipped with better brakes, even if the real life races with these cars would not be as long as the average league races in LFS.
TBO:
Like the XFG/XRG but just with quicker cars.
LX4/LX6:
I don't really know what kind of rules are used in these series so no comments
RAC/FZ5:
Mid 90s sportscar with early 00 roadster in same class.Not much modifications allowed although these cars come with some adjustability (Porsche GT3 as FZ50). Not race cars per se but but defenately capable of doing lots of laps on the limit without brake fade or overheating etc.. Built for track days but the emphasis is on daily use. Still, if these cars would be raced, they should need some modifications, like rollcages along with some added suspensions etc. options...
As to the racecars, the rules should be quite free but the cars still need realistic limits and realistic ways to adjust the set ups.