TRISTAN - There's been several people here use ATV tires with very good results. From mowing, pushing snow, plowing. With a little rear weight You probably wouldn't need to run chains in winter moving snow. The rubber in the tread of ATV tires must be much softer than either the turf or ag tires for these little tractors.
Your rear rims are 7" wide, either the 8" or 9" should fit on the wheels but about the only advice I can give on the diameter is "Try It!" The 26-12.00-12 Firestones on the OEM rims measure smaller than 26" dia, Mine on the 982 measure 26" in dia. but only 24" to the top of the tread bars from the ground.
The added dia. does help eliminate one of the problems plowing with a CC, the rearend dragging with the left wheel/tire in the furrow. I really find it hard to believe an ATV tire would be a full eight ply tire, they normally have very flexible sidewalls but they could just have very thin plys.
I'd try them, either the 25-8x12's or the 26-9X12's. If they are too close to Your fenders a Dollar's worth of 3/8" flatwashers will space Your fenders up enough to clear them, You just need them under all four of the rear mounting bolts and the back two on each footrest. The only problem could be when trying to run the center mounted mowing deck. SGT's that used the larger dia. rear tires also had longer wheelbases to clear the mower decks. Your 129 may not be able to clear the gauge wheels on Your deck with the 26" tires, maybe even the 25's.
I personally like the look of real AG tires on Cubbies. I have 23-8.50's on My 72 and the 26-12.00's on My 982. The 8.50's are 2-ply rating and the big tires are 4-ply. I only have to run 6-8 psi air pressure in the 8.50's and 4-5 psi in the 12.00's they're so stiff.
As far as what Charlie & Keith are talking about I have NO IDEA!