Sounds like. at the very least, your engine overheated and blew a head gasket. In a Kabota this is very common.
First determine why the engine overheated to begin with. A clogged radiator, blocked intake screen, failed cooling fan, are the most common culprits.
After this is determined, address your engine. You will need to remove the head and inspect the cylinder walls for scoring.
If you find scored cylinders, the block will need to be honed and new rings, pistons, head gasket, and aux gaskets installed.
Plan on a overhaul bill (if genuine Kabota parts are used) of 2500 to 3500 dollars (not including the time to remove the genset from the bus). The are less expensive aftermarket kits, you make the determination.
We have had 3 buses stop at our shop with failed gensets this summer. ALL have required an overhaul. This hot summer is finding all of the weak points.