This video explains in detail how to easily play Mario Kart Wii online again without having to mod your Wii console. This is completely legal since you need a copy of the game in the console's disc drive for it to work.

I've tested this and can confirm that it works. If you get an Error Code 60000 it simply means that you played Mario Kart Wii on the official server and your profile ID didn't transfer to the Wiimmfi server. In that case just go ahead and create a new license.