i have a 97 rinker fiesta v33 266 with a 350 mercruiser, non vortec, 260 hp and brovo 2 outdrive. the motor has been getting water into the cylinders, center 2 on dr side, to the point that it will lock up. no water in oil, after removing and replacing plugs, the rpm picks up to normal about 4800, but drops within one tank of gas 75 gal. i assume its either a cracked head or blown head gasket, hours and history on motor are unknown... i am concerned about the unrealibility, and considering just replacing the engine with a vortec longblock.... any forseeable problems? ill buy a vortec 350 with the intake , carb, and ignition already inplace,,, and also replace the exhaust logs and risers... any advice would be appreciated