Very important to find out WHY it failed, and what specific parts. This should give us a clue which direction to go with it. I've only seen ONE HST die in the 20+ years of messing with tractors, and it was due to a combination of 3 things: lack of maintenance, poor fluid quality, and abuse.
On that note, a mechanic's "fix" is to replace it as a unit and not investigate why it failed. A technican's duty is to isolate the cause of the failure, figure out what to do to prevent it from happening again, and then perform the repair (if authorized to do so)-and then educate the end user. It doesn't hurt to call Kubota either, both from the tech's standpoint and from a consumer's standpoint.