Cooling

I am interested in others experience with cooling & your solutions . My project is using a 1938 Muncie Neptune twin outboard motor for power . The fact that it is water cooled added plenty of complexity to the project . I used 1/2 of the radiator set for a 04 09 Honda CRF 250 , $70 Ebay , a very small but capable 12 v pump meant to be used for sous vide cooking ( hot water bath ) 29gph , 212 deg F , 1/4 " hose fittings , $16 Ebay , and a 4" 12 v computer fan , $16 Amazon . So far in bench tests it is running about 150 deg F in a 10 min idle . Not sure how this is going to work under load . Problem I see is the outboard wouldn't recycle the cooling water under normal conditions . I'm thinking I may need to add a couple quarts of coolant just so it doesn't get returned to the engine so quickly ? Anyone out there with water cooling experience ? Any experience out there with water cooling

no thermostat in the original system , very simple old motor . the shiny motor is a pic of a complete one that I found on google . mine was not complete , & had no spark from the original magneto ( which is the death sentence for most of these old outboards ) so I used the original points & replaced the condenser which was unavailable with a capacitor , mounted up a twin output chevrolet coil , powered by a battery . plan to charge battery with tire driven dynamo .