This is a bump, but it's the latest thread on the subject, so might as well post in it.
The summary of the current state of the restoration of RK's cars is basically the following:
1. I DLed everything that was in NFSCars and here, including the files posted by Remko, of which only three are unique (Maserati 3200GT Assetto Corsa, Nissan Fairlady Z432 and the NFS3 Eggenberger Ford Sierra Cosworth).
2. I have Alex's MM2 and Re-Volt collection, and I'd be willing to bring them back to NFS4 myself, but the state of these cars is unknown, they might not be all original anymore, although, if they're better than the originals, it might be worth keeping them in the state they currently are.
3. AJ_Lethal posted his collection of RK cars many years ago, but of course that link is deprecated. Perhaps he'd be willing to post it again?
4. There's one car in NFSCars with a corrupted download link. It's a random Toyota, not sure if it's worth going after.
5. I haven't gone through Alex's files from Christian Pittet website yet, but it should be very easy to recognize cars from Ryuji Kainoh in that collection. All that's needed is to cross the list in the archive with the list from RK's website which, to my knowledge, was compiled by FerrariMan.
6. The poster above me (Andrew L) has his own collection of RK cars and around the mid-2000s was uploading the missing cars to NFSCars. Perhaps someone could contact him again and see if he can post the files he has. Andrew was known in the early NFS modding scene and had his own website back in the day.
7. Our admin Mike Henderson has said he has the whole website backed up at one point. I wonder if he'd be willing to search through his archives once again?
8. Old NFSCars member Nemesis (known as nunyx in DeviantArt) claimed in 2007 he had all of RK's NFS4 cars and some NFS3 cars, but nothing came out of that.
9. This information I have is often overlooked, but I clearly remember Skoti posting on NFSCars that he used to have a CD with all the content from RK's website. Around late 2009 I was in an RK car hunting spree and tried to contact him first on NFSCars, then later via MSN, but wasn't successful. I remember NFS Bolide (Butch) probably had his contact, because Skoti converted some of his cars to NFS3. I can try talking to Butch about it, but can't assure any success since it's been so long ago.
Between cars downloaded from here and NFSCars, as well as the few files posted by Remko, I have 199 NFS4 cars. The quality seems to improve the later they're made. The vast majority was made for NFS3 first then converted to NFS4. His Carrera GT, Murciélago and Saleen S7 are his best cars.
On related news, since Mike Geise renovated NFSCars last year (website is really nice now BTW), most of the files in there should be safe for now, but, as a precaution, I DLed all of Felixx's cars (another famous modder in the early NFS4 era). There weren't that many, but I think his portfolio is worth preserving.