I have just overhauled my 2040, and i think it has to be pretty simple!
Try to screw of the hose between heatexchangerhousing and manifold, and fill coolant in, if the coolant comes out from the housing, then it is the connection between rubber and heatexchanger OR a brooken exchanger, which costs allmost as much as a small plane 😳
If the coolant still comes out from the manifold, then you have a blown gasket
block and heat exchangerhousing. I would say, it cannot be the head
gasket, because the thermostat will be closed when engine
is cold, not letting the oolant in the housing run out. Check it immidiately! If the gasket is blown, the oolant can run into the cylinders...