vauxhall corsa c 1.0 3 cylinder problems???

My sisters car broke down and she said that just before it happend the engine went really rattly. at first i thought the chain had come loose but when i got there i found the rattly sound was coming from inside the engine. when i took the plugs out i noticed one of the electrodes off the plug had fallen out and into the cylinder and the compression was down so it had caused damge to the valves. took the head off got the valves re seated and put it all back together but it still doesnt run right. the rattly noise has gone but its still missing and popping, the popping noise sounds like its inside the engine not coming from the exhaust. it runs ok once over 3000 rpm. i checked the timing thats ok so im out of ideas... someone told me on them engines if a piston hits one of the valves it turns the lobe on the camshaft so that that cylinder will be out of timing (if that makes sense) is there a way i cancheck this?? any ideas anyone please thanks