Missing the best model created. Where is the 4S?
Replace the 30-pin connector with microUSB, upgrade the processor, modem, add more storage. Also, use a battery that can be easily acquired (and not just pulled from other phones).
Use the existing screen/digitizer, speakers, microphones, cameras, and antennas?
No forced reliance on iOS 11.
Anybody else?
Changing any one thing (SoC, connector, etc) isn't possible because you'd have to be really good as OS hacking to add support for it, especially when it is closed sourced.
The chassis on the sides has some pretty bad gouges, but I have never had a crack on the screen or digitizer.
I did have to replace the 30-pin connector interface (it would charge and transfer data fine, but had some weird short-circuit where it thought an external speaker was plugged in, and there would be no sound for videos).
Was yours an OEM digitizer?
The 5S I have is on its third battery and the digitizer is starting to separate. The lightning port will only work with cables in a specific orientation, and slight jiggling causes a momentary disconnect (using actual Apple cable and charger).