Know a good machinist? are the gears cast or steel? If steel you might be able to build the teeth up with welds and reform them.
Have you tried Yanmar Tractor Parts web site.?
Yep, emailed them earlier today and here is the reply:
I like the rice tires a lot. In mud, snow, dirt they work great. And contrary to the way they look they ride good. Were they can be not so good is on soft lawn, and with chains installed.What size rear tires does it have?