Ok - that wasn't so hard, but exporting all the Kart reference textures manually was a bit of a pain!
Anyway, archive:
http://s000.tinyupload.com/?file_id=17170435724287890228
What it contains
KartGenerator.blend: A blender file designed to render out all the images you'll need for a Kart
MarioKartImages: The raw images for Marios Character, which I used to arrange the 3D model
NewMarioRender: A test render I did using assets from Mario Kart 8
NewSonicRender: Sonic asset from Super Smash Bros Brawl
Leaf/SSBB Sonic/Wii U - Mario Kart 8 - Standard Kart: Free online assets whicha re included here for educational purposes only
How it works
I'm sorry - but you might have to have some basic skill with Blender to use this! That aside:
I've made a very very basic rig for animating character movement, and included a basic Kart in the scene, these are on layers 1 (kart), 2 (mario), and 3 (sonic). Both are animated to go through all the frames necessary for a character, and I've used both Marios head, and the rear left wheel to align things as best I could. It's set to export PNGS with a transparent background at 64x64 into which ever folder you elect to use. There's really not much more to it than that - import your character, assign it to the rig and you're done. If you wish to import a new Kart simply hide the one that's there and use it's animation block to drive the animations of your replacement Kart.
What isn't there
I've not imported these animations because I'm lazy and hardly have time to sleep let alone machine out different animations. Were these to be suitable replacements I think it'd be cool to have a "bulk replace" option in the Kart Editor so that the frame-relitive animation images can be directly imported, replacing the prior frames, and the animation structure etc. remain intact to save on the turn around time involved with creating new characters.
Caveats
Originally I wanted to import Samus because there's a severe lack of Women in the Mario games, and frankly she kicks ass. The problem was that the rig I had made was only for "toony" characters, and so the first replacement set I made was Sonic. In the future a proper proportioned character rig might go a long way.
Wheels
Aren't we supposed to cut them out or something? Is that a secondary animation set or something like that? Oh well, guess you can't fly without crawling first
EDIT
So I found some spare time while I was doing a compile and I put together a more proportional rig for racers, and also setup an empty for parenting Karts to, and two for the front wheels. The empties for the wheels don't have a location set in the keyframe so they can be moved to suit the position of any wheels you wish to add. I've still not even tried importing any characters yet, mainly because it's a time thing, and the setup is still horribly simplistic. But long story short: the following file contains the new setup blend (sans the extra stuff pertaining to the models) and a full render set for Link.
http://s000.tinyupload.com/?file_id=57369350847955236228