There is a YouTube channel called Rebuild Rescue that is completely rebuilding a 401.My oldest just went through transition training to add the old Cessna 402 to his qualifications. Good thing for him that he got his instrument rating initially on the old 'steam' guages, as they refer to them now. The Cessnas in small airline service today have been in service for decades and still use the older avionics.
If anyone remembers the old sitcom 'Wings', the airplane in the opening credits is still in service today.