Don't worry about it too much, no need to perfect everything at once. Really nice work so far on this project!
For the sound effects, you can take them directly from the original game. From your Lego Racers program files, extract the LEGO.JAM file (LEGO.jam File Extractor - Modding Tools - Rock Raiders United), and inside the GAMEDATA/COMMON folder there will be all the sounds you're looking for. They are in .PCM format, Media Player Classic (Home ยท MPC-HC) opens them perfectly fine, it's up to you if you want to convert the files to a different format.
For the animations, I think you'll need to reanimate those in Unity. Take a good look at the original and try to mimic it.
Good luck! I'm curious for more gameplay (especially AI)