GoodGen 3.21 isn't too tough to find, but yes, it would be nice if this mod worked on a standard copy of Rock 'n Roll Racing.
Can we talk about how nuts this mod is, though? The planets spin in the character select screen, more characters have been added to the cast, Larry Huffman no longer cancels out the soundtrack when he talks, up to six cars can race at once... it's a pretty significant expansion. Heck, you can listen to the original soundtrack with lyrics if you have a Sega CD.
I don't know how the hacker managed to do all this, or why he thought it was so important, but it's nevertheless really impressive. It makes me wish I liked Rock 'n Roll Racing more than I did, because for devoted fans, this hack has got to be incredible.