FWIW, there are still many active N2k3 sites; a few are generic and you can find most mods and tracks there, many are oriented around one mod and have many skins, reworked tracks, and online leagues for that mod. The two best sites for modding info, in my opinion, are not accessible - TheUSPits forums disappeared (though a small page of downloads remains) and while you can visit Stunod Racing much of its forums are "you must be registered to view this" and going to the registration page gets you "sorry, we are not taking registrations".
If your interest is PTA or GTP you will find little beyond skins as these are not "NASCAR". There was a good bit of info and downloads for the GTP mod at SRMZ (if they ever return); I have about 200 skins covering LMP and GrandAm, plus some fictional ones from defunct online leagues, and the high res templates and steering wheels. Interest in the GTP mod waned when iracing opened their LMP series, though there are still a couple of online leagues.
There are also many new NASCAR physics out there, along with a physics editor. I've no idea if these are "legal" ...did Kaemmer lift his ban on editing the n2k3 exe?