We were driving on the freeway. I started hearing a knocking. Then the car died. No dummy lights came on all gauges were normal. After 30 minutes car started. Still could hear a knocking and started to over heat. We replaced the water pump and the radiator. Still knocking and overheating. Took to mechanic and they said we need a new engine. I was thinking a new oil pump and possible thermostate. I could even buy a new head gasket but I don't think it needs a total engine. Is this a good estimate?