Tamiya sells some mini 4wd monster kits and they have pretty big tires. They're not that expensive. I put some of the smaller versions on my xmod and mounted the wheels backwards with washers... Now I can run on my carpet at least
Plus, it's wider (offset from backwards mounting). I'm guessing you are going to put a long-travel suspension on yours? This thing sucks off road(w/o AWD), not enough susp. travel.