Great little mario cart engine! The balance was good too with lots of nice touches.
My main advice is to focus more on the feel and control of the cart. It controls super sticky and needs to be loosened up a bit. For example in mario kart you can drift around corners.
I didn't mind the green tortoise shell power-up but I think if I had a choice I'd rather use the 2 buttons for boost and jump. These kind of movement options allow players to make much more fun choices about how to navigate the track. The tortoise shell is a less interesting choice and skill check.