[ndutton]

As for normal engine wear, it's my opinion that the engine will let you know. One telltale sign of progressive internal wear is blowby, that annoying smoke that comes out of the crankcase breather at the slash tube or in later stages, the oil fill cap and stinks up the cabin. Many owners add a PCV system to manage the smoke and stank but the existence of the smoke is an engine looking for attention. Another wear indicator is compression. It's easy to test, probably good to check it annually and record it in an engine log. With that you can identify a deteriorating condition if one exists and proceed with remedies when the time comes. A third indicator is falling oil pressure.

A lifetime of raw water cooling is a different beast. It is simply a matter of time until something rusts through and if that something is the block, it's a death sentence to the engine. Any rebuilding plans evaporate at that point. If you have budget available, the sooner you convert to FWC the better. It will arrest the rust instantly. Even if you replace the engine the FWC system is transferable.

As for the age of the engine, you report it has been 30 years since rebuilding. In comparison mine is 42 years old and as far as I can tell, has never been rebuilt yet still runs smooth and smoke free. The day will come I'm sure but at this time there is no indication it will be any time soon.