Just replace kart.dff and kart.txd inside gta3.img archive or use modloader.

Check this mod here:
https://www.youtube.com/watch?v=CEvworiYQCA

And subscribe to my YouTube channel for more great mods:
https://www.youtube.com/channel/UCHWvKWerWjylBHbgKjGy71g

Here you can download more mods:
https://www.gtainside.com/user/Dragomazing