If you still have the original Serck heat exchanger, it could be removed and then the tube stack could be descaled and pressure tested by a radiator shop. Secure your
raw water valve, drain the
coolant and
salt water, loosen hose clamps and it's held on by two brackets. Radiator shop maybe charges $50.
The main h/e has end caps sealed by rings on the raw
water side, and paper gaskets for the connections on the
coolant side.
Parts are available from Trans Atlantic
Diesel. The Serck heat exchangers are quite expensive to replace, thousands of dollars plus time and expense to ship from
England. Most people replace with a generic cupro-nickel h/e available for the hundereds of dollars you stated in original post if replacement is needed.