http://www.millertire.com/categories/lawn-garden-tires/
Maybe you could use something like a 23 X 9.5 X 12 turf on it with chains. I'd call Miller and try to find out what they think.
EDIT
Call Patton or Cub Classics and find out if the have a set of wheels and tires off something like a 2135, they are 20 X 9 X 8 pretty sure they would bolt on your tractor.