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
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.