Here's the deal-
I suppose the OP asked around locally already about a used or new wheel, otherwise he would not be asking world wide where he could find one. The 12" 4 lug is no longer supported by Kubota,
He may find a 3.5" or 4" X 12" wheel but it will be a spoked trailer wheel and it will be 4on4, Kubota 4 lug is 4 on 103mm. The 4 on 4 will not work with standard lug nuts, nor will it mount to the Kubota hub without a 3/16" or 1/4" adapter plate (of which I never mentioned),
He cannot mount a 5" tire to a 5" wheel and further I'll bet his current tire is NOT OK.
His other wheel is probably NOT OK either, if one broke, you can assume the condition of the other.
You got a problem, now you need to come up with a solution. Most folks are spoiled, they think everything should just be handed to them........and cheap........or free.
You have a 20 to 30 year old tractor that is barely supported by the manufacture.
If you want to fix your problem you'll likely be buying new tires too.