I want to make a racing style game, but I can't find any car related assets besides the racing kit (which is what I got the idea from). Is it fine if I create my own car from scratch?

Nope, you'll have to use the assets provided.

You can find top-down 2D cars here: https://kenney.nl/assets/racing-pack

or 3D ones as an exclusive in Kenney Game Assets 3: https://kenney.itch.io/kenney-game-assets-3

As a tip; as the theme is 'time travel' you might be able to replace cars with medieval siege weapons or spacecrafts.

Welp. I'll have to change all my plans.