If you have gelatinous white material in the thermostat housing, I think there is a good possibility that it will also have gummed up the heat exchanger and perhaps other parts of the cooling system. At the very least I would do a thorough flush of the cooling system and add new coolant and thermostat.
But if it were my boat, I would drain the coolant completely, remove the heat exchanger and take it to an auto radiator shop for cleaning/servicing (about $80 - 100, usually one day turn around), check all the cooling system hoses and clamps for softness, cracks, rust on the clamps, add new coolant (diesel coolant, not just regular automotive), replace the thermostat, and then bleed out any air bubbles to avoid engine overheating. This is normal maintenance that should be done every few years anyway, so it will be one job done, and minimize the likelihood of your engine overheating which can be a very expensive fix.
Removing the heat exchanger is not complicated once the coolant has been drained, but it can be awkward to reach all the cooling hoses and they can be stubborn to remove from the heat exchanger. There is a tool one can buy that helps this, but most of us just use a screw driver to gently pry the hoses off.
Frank