I am looking at a 1992 Cajun 1850 bay boat. It has a 1992 Johnson 115hp motor on it. It has 130-140psi on three cylinders and about 40psi in the other. Assuming it's not just a head gasket, I am trying to figure a "worst case" estimated cost. What do you guys think "at worst" this couls cost? Also, will other power heads fit that lower unit?