The engine with the longest life and production run is the Lister CS.
Production began in 1929 as the RA Lister CS 1/5 and ended in Dursley 1987 with the CS 1/8.
Clones continue to be manufactured in India, Pakistan and Iran.
If it comes to being the last engine running it will be CS, and it will likely continue to be manufactured for the next 20 or so years.
Top that Honda.....
Some where out there is a CS that has never been rebuilt in 90 years, and it is quietly still turning a days work.....