AussiePorsche

Hi from Perth Australia, I have a MY06 Cayenne S V8 which at 42k miles which has lost its coolant (suspect plastic pipes) but it also has had a heavy valve impact on piston number 3 (bore scope checked) in addition compression is down on number 8 cylinder. The codes identified multiple misfiring and camshaft coolant temp in excess of 142 degrees. My customer says the coolant warning light came on and he drove a short distance then the engine stopped thats it. The engine turns over with ease by hand and starter. No one has rebuilt a Cayenne engine in Australia yet with the chosen option a new engine. Can someone please advise if they have seen this outcome before? also If the cylinders are not scored is it feasible to rebuild the cylinder heads? are parts available?
Porsches reponse is to just pay for a new motor but if the cylinders are ok at this milage I would like to rebuild. Your thoughts and comments please