I think you could find more people alive today who could engineer a steam piston than could build a wooden ship or make straight lumber with hand tools in the quantity need to build anything significant. You would not need to know how to forge high carbon steel just how to heat it and reshape it, it is lying around everywhere.

I think with a little help from someone with some mechanical engineering skills and few books that we could scavenge from libraries and other places I could convert an diesel generator to run off of a steam powered engine or possible even a water mill. I know I could repair modify electronic devices to keep them running for a while. You see them driving cars but never using heavy machinery. I can run a backhoe and if I a couple of hundred to steal parts from I could keep it running.

Black powder would be easy and the ingredients would be very plentiful in the right locations. Smokeless powder on the other hand would be difficult and most modern guns are not built handle black powder but it would still be simpler convert the tens of thousands of guns that out there than it would to start from scratch.

The bigger problem is the next generation. Most everyone under the age of twenty is dead and they are not having enough children. If they don't stabilize and start breeding they could have a population crash that would basically be a second apocalypse,.