This is a 2000 SUV with ~100 hrs. The motor was running really rough that we had to tow it in. While at home, one of the spark plugs (the 3rd one) was very hard to remove, and was melted. After I got it out, I could not thread in a new one. I also could see a hole on the piston through the plug hole. More likely a new top end is needed. How easy to do a top end? Do I need to remove the motor from the hull to work on it? Should I replace all 3 pistons or just the bad one? Anyone have or know where I can get a procedure for the job? I guess I will go "premix" after the repair. BTW, the motor can turn over with no issue. Thanks much!