jughead

Hey everyone, jughead here, i have a question please. I bought two nitro rs4 rtr3 cars, one for me, one for my son.Anyway,we been runninr them for a couple weeks now and my son ran his with a friend today and said it died and would not start.I took a look and the exhaust manifold screw was back all the way out.I retightend and got the car started but it runs really rough and minimum power.I then noticed the inside top of the head is a brown burnt color and when i pulled the glow plug out, brown and burnt lookiing downinside also. It is 15 fe motor any help would be appreciated. thanks

JUGHEAD