First of all, Marine engines are cooled by outside water sources instead of coolant. Any particles or organisms in the water are also in the engine, if left sitting for any length of time they are prone to accumulate deposits to form blockages in the block. Drain and flush all coolant from system, remove the thermostat, fill engine with 1 QT of a product called CLR and the rest water, circulate for 2 min., let stand for 24 HRS. Drain and flush. Go to your local junk yard and pick up a Rad. from a Caprice Cop Car with the shroud and mounting brackets. If you live in a fairly warm climate, replace the thermostat with a 180 degree model.