Fabia 1.2 12V HTP Petrol - Engine break down

Dear Members ,

Need your help to understand the root cause of my Fabia engine break down while driving the car.

I first got low oil alarm , stopped the vehicle and check the oil level , found intact . Then observed that main crankshaft pulley centre nut in loose condition , tried to start the engine , it was cranking and not starting . Took to the dealer and they found that valve is broken and created dent on pistons . Now engine is down .

Before this incident , I observed the rattling noise from the engine at cold start , reported it to dealer twice and they retuned car to me by doing some work . Please find the video of noise.

Please help me in understanding the cause of problem faced , i.e engine is down , dealer is advising me to go for new engine... 

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