Hello,
Take some 1/8" or larger, depending on engine size, and heat it up until it glows red with a propane torch. Then slowly move up the length, making sure you get the whole thing gloing red at some point. This is called annealing, it softens the brass. Wrap 3-4 turns around the cylinder head, and solder the brass tuding together, make sure it is tight on the engine for good heat transition. Then, hook up the water inlets to one side, exits to the other. You also have to get a flyweel to facilitate starting and running, as part of the process in an airplane engine uses the propeller to make the engine turn over. The boat propellers have low mass combined with high resistance, so the extra mass of the flywheel keeps everything turning. Octura is a good brand, and I believe they are still in business. Cool clamps do exist, but they are not as good as the brass tubing method ( less surface area.) A heatsink is not the recomended method to use in a boat. Boats are usually enclosed, with very little or no airflow. It's also a small space, which means it will heat the whole but up quickly. Water cooling is easy and inexpensive, so why not go that route instead of creating porblems?
All this info on converting airplane engines came from a book published a few years ago. There was a point when no manufacturers produced good marine engines, so everyone had to convert from airplane use. Nowadays, however, there are quite a few manufacturers making marine engines. O.S., rossi, K&B and zenoah all make good marine engines. Just pick one suitable for your boat, hook it up and go shred some water!
Kurt