Welcome to the forum fella.
I understand that this came out of a yacht and was running sea water for cooling, is that right?
I see that in your picture there is a marinised style manifold on the engine now, is this your new cooling system?
What do you want to do with the engine, back into a boat or used on land and at shows or on a property?
I converted a small 3 cylinder Daihatsu engine over to run a Hamilton Jet in a mullet boat. The engine coolant went through a 4" round stainless steel tube and sea water was pumped through the inside of that big tube through some #/*" tubing to cool the engine coolant. The sea water was circulated by the Jet propulsion unit.
If you wish to run this engine on land again then it can be kept cool by running your engine coolant through a radiator, or via a large water tank similar to a 50 litre water tank and allow it to thermo syphon. In this case the bottom of the tank mush be above the engine in height.
hope this helps in some way, if not ask again.