Wow. You have a Grasshopper? I haven't seen those in a long time. They're ugly looking but boy do they work good.
For the drive wheels, you need 21x11.00x8 bar tread or 20x8.00x8 turf tread.
For the rear wheel, you need 13x6.50x6 rib tread. Keep in mind that they pneumatic tires.