The limiting factor isn't generally crank/rod/ or other major
component fatigue (especially in those bullet-proof O-320s)but is
probably corrosion and cam/lifter spalling. Oil analysis might catch
either one. I doubt the factory has any info beyond that they would
or even could release.

From a strictly cumulative fatigue analysis standpoint there could
have been an innocent relatively short term event in the engine life
such as simply max throttle on a bitterly cold winter day that has
done most of the fatigue damage. Or it could simply be long term
operation at a wrong RPM where the crankshaft & prop are resonant.

This type of "abuse" exists, but the wise ones (i. e. the certifiers)
have decided and found that there is enough metal in this engine that
it seems to be able to handle it. Hence for example there is no
prohibited RPM range. Certainly the operating experience of a large
fleet helps reinforce this. Whether that is true for precisely all
supposedly identical engines and all conditions is impossible to say,
but the history on this engine series is probably the best in the
business. But engineering is an art, not a science. As one
(mechanical) I can understand why the factory is reluctant to say to
keep operating it.

I have an O-320 E2D in a 172M last assembled over 29 years ago, & now
at 1700 hrs TTSN. It has been opened only slightly for the oil pump
gear AD, but otherwise it has shown no deterioration in compression,
smoothness, or oil consumption over its lifetime. I'd like to think I
have run it pretty carefully, but I realize I am running a slight
chance. On the other hand, seeing the occasional AD show up on newly
manufactured parts scares me too. The comfort I have is that I
personally know the entire history of its operation. My decision is
to keep operating it.

Our IA once mentioned a Citabria in fish spotting duty out over the
Atlantic that had 3800 hrs on it without overhaul.

Unfortunately the limits of technology has to make it your call. But
others are doing it too.