Whether there'll be enough salvageable generators, etc. will depend upon what "S" hits the fan. Massive solar flares may render almost all generators and electrical devices moot. While it might not be too difficult to contruct a crude generator using wind, animal, or water to turn the armature it will be difficult to modify many devices designed for alternating current to function on direct current.
Homemade diesel might be an option in the right circumstances. A lot of the agricultural equipment used today would take a lot of modifying to use on anything else except tractors with 3 point hitches. Plus, this stuff wears out and is dependant upon a manufacturing and distribution chain that may not be restorable (at least in our lifetime).
Will there even be people with the skills to do these things when so much of the population is dead?
I agree that salvaging may be an option in a future scenario but it's possible that it may not be. It doesn't hurt to have all the bases covered.
Again i find myself in the minimalist category. i have a hand drill, a number 5 jack plain, a draw knife, a reloading press some aggressive bow saws, files, and an ax with splitter. if TSHTF no one will follow the sound of a generator to my front door. F that im leaving my house and living in the woods. what i just listed is more than i need to make a temp shelter and complete a 1000 square foot cabin fare enough away from the road that no one comes looking for me. Over prepping for a long term situation and the attention it draws may constitute the greatest danger a prepper faces.