The engine coolant pump maintains a suction to the coolant tank. So if the thermostats are closed there's still a flow thru the engine if the normal bypass is used. The engine coolant pump should maintain circulation thru the heater without a separate circulation pump. The speed the coolant passes thru the heater may need to be regulated. Too fast and it doesn't pick up enough heat. Usually you have a valve to regulate flow.
When I build something and don't know what to expect in heating value, I would add a tee with a thermometer on the in and out of the heater. A heat gun on bare pipe also works. I've got a number of homemade additions to stoves for heating my hydronic system and hot water. I have a pellet stove and wood stove with water coils that are tied into the boiler and produce enough heat to keep the diesel burner from coming on. I did this the last time diesel was $4/gallon. Stoves also have pressure relief valves in case of a flow restriction and coils make steam. My galley stove is diesel with a coil that heats a 50 gallon water heater. The pellet stove runs 24/7 in cold weather and can keep the whole boat warm. The wood stove is mostly used to get rid of all the packaging that come with groceries. I did heat with wood 2 winters, but it's not practical. Sometimes in the fall, when anchored near a lot of good beach wood, I'll cut a couple cords for winter. Carrying a couple 40# bags of pellets at most is a lot easier than several arm loads of wood across icy docks.
