Interesting :)
Nothing exceptionnal here, yet a very interesting start.
Simply transform the background into tiles (using a X and a Y array), give a kart-blocking script to the edge tiles and a kart-slowing script to the grass tiles, and that would be worth a 10/10 simply for the idea of remaking mario kart.
I only give you a 5/10 as a rewiew, because, honestly, come on, we both know how quickly this was done, yet I'll vote 5/5 because I like the idea and I know you'll go far with it :)