Search the Internet for Industrial Kubota Engine dealers. They have better pricing for genuine Kubota parts. You will need the real Kubota part numbers since they do not have access to AG/Construction side parts books.Yes, sorry I was unclear. basically Kubota and their dealer network want to sell new or authorized rebuilt engines. as far as amazon kubota parts, I have found them to be poor quality and poor fit limited with no availability of oversize/undersize parts forcing you into additional new parts. basically as Jon said, you can rebuild your own engine but at 2500+ hours you are better off getting a new or authorized rebuilt as it will be about the same cost as what you will have in parts and machining work (if you can even find anyone to do just the machining work on the kubotas).
As far as fuel economy, as Jon stated the diesel was better than the gas for sure. but I use several ZTR's now and they mow much better, faster and with less fuel than the SGT did. my SGT is on snow blowing duty only now. the only Kohler I own is in a 106 FEL/tiller tractor.
to me, in the end, all the kubota powered tractors are the best that Cub made (including the yanmar tractors) but the plastics are easy to break so buy the bumper and/or dont run into stuff. I personally would have a kubota diesel repowered 982 if I could.