Sketchuping a head seems like an easy thing to do, guess that's where I'm at next, I am tired of lookin at Mario's ugly mug! heh! Then of course the rest of the body parts, and I guess I should re-do the rigid bodies for the body parts, and of course some actual vehicle skins, sigh, so much work for a total Slacker!
Plenty to be getting on with then.
A quick point for the head - the best way I've found to make sphere's in Sketchup is to use the Sketchy Physics plugin. (Not too hard to find on a search or using the SKetchucation forums)
It does a better job of making sure it's all closed (hence proper collision and shadows) than doing them yourself.