I think I may be able to find a tire size in my original Japanese manual that covers the 2X02 model tractors, but it might be a while until the manual and I are together in the same place. I'll have a look when I can and post back.
There is an important relationship between the circumference of the front and rear tires if you have 4 wd. The front wheels need to be just slightly pulling the rear ones and this requirement is incorporated in the design of the tractor.
Getting the original info on both the front and rear tires will be important to not damage the drive train by changes in tires.