Doesn't take much to ruin an engine, bend a valve or worse. I would not turn that engine over until you find the part. IMHO anyway. I remember once I small screw fell out of may hand, spend the day looking for it, couldn't find it and thought it was offered to the garage floor gods, I mean, no way could a small screw jump from two feet away out of my hand and down an open intake tract could it? But I ran a magnet probe down there anyway, nothing, I mean what are the chance that the screw fell out of my hand, it the intake tract AND the intake valve was open enough to let the screw in, right?
Well, a bent valve, damaged head and damaged piston later I came to the conclusion I should have played the lotto that day as the chance gods were in my favor. Moral of the story, find the part!