All the water cooled model motors I've seen use the same parts as the air cooled ones. There have been a few instances were application changed some things. Like OS had the old .32 in helo and aircraft and other than heads they gave the airplane version ABC and the helo version a ringed piston. Water cooling would be beneficial to keep a more consistant temperature throughout a flight. Helos used to have cooling problems before efficient fans and big heads came along. Designers could have just as easily gone to liquid cooling though a radiator of some sort would have probably been fairly expensive.