Take an oil sample and send it out for analysis. The dealer has to have a great hearing to hear a high-speed knock in a diesel engine. I would be inclined to use the tractor for another 10 or 2o hours and then do another oil sample. If the sample comes back with bearing material in it, then you know that you have a problem. You might have taken some life off of that engine, but that doesn't mean that it is junk right now. I know of more than one person who blew a fan belt on a Corvair engine and overheated it, and some people said that the engine was junk, but they put another belt on it and drove it for many thousands of miles with no ill effects. Hell, if it turns out that it runs well now that there is new oil and filter in the engine, maybe it will be best to trade it in for a John Deere or another Kubota. Just don't go to the dealer who told you that you needed a new engine. Just make sure to put some months and hours on it before trading. If you buy a new Kubota, buy the KTAC insurance!