Can probably fix it with a heli-coil. Get a new stud and a heli-coil set for what ever the thread size is. Put some red loctite on the threads of the stud when you install it. Check that the pushrod is not bent. When you get it back together crank the engine over by hand and watch that the valve is opening and closing smoothly and not hanging up.

Change the plugs, should go away. My dad had the same problem and come to find out it was a cracked spark plug.

Looks like I need to replace the head. Helicoil did not work, don't need to go into detail, sometimes things just don't work out. Anyway, Have found a new and used head and not sure what to do. The new one comes with the valves already installed. Question is can I replace just one side? Maybe a stupid question but..Also how far should I go with this engine? Should I be considering replacement? A new engine with more horsepower would be nice considering the weight of the boat 6400 lbs. dry. getting a little frustrated and need some suggestions. HELP!