Have you looked at 11L-15 tires or other tires that are used on most hay balers or for floatation. Also, since it is 2wd you could put pretty much any tire that would fit on the front even if it is one size bigger or smaller without throwing the center of gravity of too much. If you go with 16 rims it would shift the center of gravity more toward the rear of the tractor to give more traction but the 3 point hitch would lose an inch or lift. If you find the right bolt pattern and offset on the rim, you can also use wide 15 truck tires. I have used them on hay trailers and that has help keep them from sinking into the mud a lot.