Best way to ruin an old Atomic-4 is to flush it with muriatic acid.
An A4 that old is probably somewhere near the end of its life due to
internal corrosion. Use a non-acidic descaler such as Marsolve or
Rydlyme ... it wont disolve the base metal.

How to check if the engine has enough 'meat' left in the block to be
worth bothering about: either you or the surveyor/mechanic remove a
single (or more) exhaust manifold bolt, then go in with a dental pick
and count the number of threads remaining in the block. If you can count
at least four threads in one of the exhaust manifold attachment bolt
bore holes, then the engine has not rotted awawy internally and has
sufficient 'meat' left.

To prolong life, seriously consider to add freshwater cooling, even in
fresh water. With fresh water cooling, the engine 'cooling loop' will
contain antifreeze with its associated 'anti-corrosion' compounds, etc.
The important thing here is that you will never (when winterizing) have
to drain the coolant (other than the raw water side). Draining and
letting an A4 'air-dry' internally will promote HUGE chunks of rust
platlettes to break off, which will eventually clog the exhaust manifold
and engine internals ... keep it WET and dont let it dry out internally
- for long life.