Mario Kart 7 romhacking, what's possible?

Hello.
I recently found out that Mario Kart 7 can be romhacked. But I don't know what's possible via romhacking. I would like to edit the tracks and make custom ones, replace the music, etc...
I currently don't have the game, but I do have enough eShop credit to buy that version.

If this has been asked before, please point me in that direction.

Thanks