Engines can be modified to run on a lot of thing you can grow and process on a small farm. Maintenance would be an ongoing skill needed, so the role of mechanic

Engines can be modified to run on a lot of thing you can grow and process on a small farm. Maintenance would be an ongoing skill needed, so the role of mechanics and electro-mechanical knowledge would be highly valued again. A harder challenge would be stripping out all the ECUs, computers and security crap to make things "just work" again. All civilisations have been able to maintain their technology, but I think we are in an unusual and tremendously precarious point in history in the early 21st century. It seems far more likely that a loss of control over our technology will cause an apocalypse than that an apocalypse will cause us to lose technology.