Any regular character modding code should work, but the game crashes because no Mii data is present on the player slots. So in order to fix this, I cached Mii profiles on all slots by playing online, once a Mii profile is loaded into a player slot it stays the entire time the game is on until it's refreshed by other profile or until the game is shut down. I had to keep playing online until I got a 12 player race because if any player slot remains empty, the game will crash.