I have a 90 Mariner that I bought new in 1987. It been a great engine, never given me me a moments problem. but now has devloped a water leak around top 2 spark plugs. looks to me like a gasket problem around the water jacket. For all the world it looks like the waters coming from the spark plugs, but I know that not right. The engine runs great! Question is how hard is it to replace this gasket if that what it is? And should I replace anthting else while I have it down? How much do the gasket usually cost?