Once you over rev an engine and especiall an interference engine like these are it is very easy to have piston to vale contact. That is a noce way to say you had the valve nased into the piston and now you will have a very big hole in your wallet. . First try and do a compressionest and pay particular attention to number 7 cylnder and if it is down then you will need to do lots more work. But if you want to be extar sure before you spend then go buy the self lited and screen optical camera for diagnosing thinsg. The can be fedd into the cylinder and you cna then see the piston top and the vales and all the rest on the small 2inch screens. These have built in lits and so they can be great at seeing the damage that has ocurred without taking it all a part. If you can not see any damge and the compression is ok then you will have to move on to other things. But oil in the cylind and on the plug is not a good sign if you ask me. So do this right and set out you plan of attck and follow it. If you miss shift from time to time and you do vae to take a part the engine then rebuild it to withstand the rev conditions with say better valves springs and always better camshaft gears from the stcok ones that seem to be made with the old saying (that is close enough).