At the risk of sounding like a smart ***, expose the plug and see which metric wrench/socket fits it. If you don't have metric tools, buy a cheap set that goes up to around 20mm and there will be the right size included. If you don't have the tools already, they will come in handy in that you own a Kubota machine.
HF has the Metric impact sockets on sale for $19.95 a set and they work non impact as well. Get a 20% off coupon off the net and get them even cheaper.
I like the lifetime replacement guarantee too. You break one, they replace it. I've tried (to break one). So far, no luck. I'be split Snap-On and MAC sockets but not HF.
Just did my snowplow cutting edge and getting the old one off was serious impact wrench work. My IR Thundergun split the Snap-On and MAC sockets, not the HF cheapo ones.
I think the drain plugs are 22 mm if I remember correctly.
I try not to use a West Virginia Socket set on any hex nut or bolt. Fine for a junk fastener, not so good for something you want to re-use. Vice grips work too but the same deal applies.