To my knowing there is no mod and probably never will be. The game UI wasn't designed for 16:9 or other aspect ratios.
I just launched vanilla in 1920x1080p and didn't see any ui stretching.
Do it the other way around. start in 4:3 aspect ratio (like 1024x768) as the game was original meant to be played (~2004-2005) and then you see how it should look.
At least if you switch later to 1920x1080p again, you see the difference
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adjusting all the vanilla UI textures in aspect to fit the 16:9 ratio is an option, but that's not what I would want to do. Oh, and that must be done for all race mods as well. The results could look worse then the stretching we see now I can imagine.
Edited by mdcertainty, 24 September 2019 - 11:41 AM.