2003 Hyundai Tiburon Running rough

Hello my son car over heated going down the road he let it cool down and put water in it and drove 5 miles to home.I found a leak in the top of the radiator leak a lot of water out.I fix the leak and put leak stop in it. The car ran good for a few days and it started losing power up hills and and the motor was running rough.I though it migh be the caltlickconverter the man at the muffler shop check it and said it was ok. He seems to thank its a vale.I put some new plugs in it and notice the end of the plugs wires were shorting out that goes inside the heads connects to the plugs I put ele tap around them and the car ran just a little better not much. The driver side tail pip is black looking the other side pip is clear. So I don't know if its a vale or a head gasket or timing the motor miss a lot. Do you have any ideal what is the problem and how I can fine out if its a vale or head gasket.
Thank You
Mr. Powell
God bless