help my old seagull

[alan stewart]

hi everyone just joined i picked up a seagull engine thrown in on a boat jumble owner who by the way was a jaguar mechanic said it runs but when in water it gets into the piston serial no is s36144 i think its a silver century from the sixties put clean fuel in has good compression and checked spark it starts but within a minit excess fuel from the block runs down the bottom of flange chrome bit and out of the holes of the water intake then goes rough and dies it pumps water intermittently checked impeller metal one seems o.k is this cracks in the cyinder block or possibley warped head any advice please