It sounds to me that you have an injector that is leaking or stuck. the rise in engine oil level, if no water was present means excess fuel is getting into the crankcase past the rings. Maybe you should charge your "Friend?" for the damage done. "Friends don't loan Friends expensive Kubota products".
Try to isolate which injector/cylinder that is making the knocking. When you find it, pull that injector and have it pressure tested. I would mark it and also have all of the others tested too. Hopefully your so called friend didn't suck enough water into the engine to bend a rod or crack a piston.
Try to isolate which injector/cylinder that is making the knocking. When you find it, pull that injector and have it pressure tested. I would mark it and also have all of the others tested too. Hopefully your so called friend didn't suck enough water into the engine to bend a rod or crack a piston.