Pull it off the machine , bring it to any Kubota dealer and they can show you a breakdown of all the internals and price replacement parts, you may find something on ebay as to a used gearbox - I'll be willing to bet a new replacement box would be costly .
Going to need more information like model , year etc. If it's the one I'm thinking about , it can be rebuilt - I've done a couple of them - Parts have to come from Kubota - gears, bearings, seals a shaft if necessary.
post a few pics as well .