As these cars approach 40 years old, it’s the years and not the mileage that cause more problems.
There are no computer codes on these cars.
Your answer lies in this thread somewhere:
https://forums.pelicanparts.com/911-930-turbo-super-charging-forum/547032-sticky-common-answers-why-my-car-doesnt-start.html
A systematic approach fuel, spark, wiring will eventually uncover the issue. The ass-kickers are when it’s intermittent… that’s usually a dirty electrical terminal somewhere (often under the strivers seat on the speed relay.
Good luck with your analysis and come back and tell us what you found. We’ve all been there.