It is fair to assume the car was good value, you had the opportunity to drive the car, lift the bonnet, explore the ins/outs and drove it for over a week. Whilst hard to take the cylinder head gives no advance warning other than perhaps and not always some oil resting on the road under the engine overnight but at that you would smell that oil as if burns off a hot engine.
It is a stretch to say the seller sold you damaged goods, you bought at face value privately, you have been unlucky but that was a risk you accepted or you would have paid more for a similar car at a dealers who would have provided a limited warranty.
Get it repaired as best you can and forget about it after that, you have no recourse to the seller other then perhaps some goodwill gesture.