2003 Dodge Caravan no knock sensor, my engine toast?

I purchased this vehicle two months ago. The engine has been making the marble knock sound when warmed up at idle. Engine sounds fine when cold at first start but as it warms up the noise becomes noticeable. I can't hear any knocking when I'm driving. Only when the engine is idle I can hear this sound and it is getting louder. No loud knocks, just sounds like more marbles. I have done research on engine knock to find out newer engines use a sensor for ignition timing. I thought mine might be bad so I wanted to check it. When I crawled under the van to remove the knock sensor, there was not one there. The hole had been plugged with a threaded plug. I then at that time came to the realization that this engine is possibly ruined. I do not know how long it has been gone. The check engine light is not on. I have noticed when I step on the gas there is no strong acceleration, it's slow to respond. Please tell me your opinion, when I put a knock sensor in will the timing be re-calibrated so this noise will be gone? Or is my engine beyond going back to normal and the damage has been done in which the sensor will have no effect?