Milling the head won't do much for horsepower. It was probably done, because he overheated it, and nuked the head gasket. Cubic inches are really the only way you can get more horsepower. In this particular year motor, you can take a 25hp motor and increase it to 35hp, by changing the intake manifold and carb, but milling the head won't make it a 40hp.
Unless you are into competitive racing, that is to say a real race boat, don't bother. It is hard to tell the difference five horsepower makes. And you may actually make it worse.
Unless you are into competitive racing, that is to say a real race boat, don't bother. It is hard to tell the difference five horsepower makes. And you may actually make it worse.