Honestly, you should've gone ahead and released it with the proper credits... Today the "Fair Use" doctrine is developed enough to cover such cases.
You could post the author's words here for us to take a look, although copyright laws supersede any limitation imposed by the author where applicable.
The old draconian rules in the NFS community were imposed by certain people because:
1) The games were relevant in the modding scene.
2) People were taking others' cars and adding FnF-style parts, etc. to them indiscriminately, as well as releasing the cars for other games under their own name. Authors didn't like it and got those people banned. This no longer happens, as the FnF fad died out a long time ago and the bulk of modding today is stolen Forza content.
3) I've had this idea for a while that people modded NFS not really to add content to the game but rather to build a portfolio for when they eventually turned professionals. Which is why the vast majority of highpoly NFSHS cars are unusable in the vanilla game, and many famed modelers of the era only have one or two releases for NFS games despite having had several ongoing projects back in the day. If you're a hobbyist looking into starting a professional career as an artist, you're going to want to protect the stuff you built.
I sincerely believe that treating the work with respect and not turning it into an Audi TT Quasar-like monstrosity (the magna opus of the infamous Kremit De Frog) makes things ok here. If the author finds out and wants it removed, then have it removed.
Recently a similar thing happened in Assetto Corsa. the_meco and his team bought a Lamborghini Miura model from a website, intending to create a mod out of it. Eventually it became one of the game's official cars, with the blessing of the original artist.