Thanks phil170258. I think it's fine to navigate by GPS as the aircraft you are planning will be 'old school', with special wiring to power the GPS unit. Naturally, this would not be built in but a small unit fixed on a stalk or bracket to the edge of the panel or a window? The folks in here can guide you on that. Likewise, the radio and comms would be basic - as long as they meet all current regulations! On the ground, you could seek to minimise use of computers. At departure, he might be asked for his load manifest to be emailed but all he has is a hand written page? As mentioned, I am pax and not familiar with the detail there.
Cowboys might want to try and use a cell phone for communicating with his reception committee as he gets close and that gives some opportunity for it to work/not work. Or to receive SMS whilst flying and then the same as in a car - don't try to send txt messages when you are the sole pilot! Does he have to ask a pax to help? As long as looking at the small screen of the phone does not add to ill feelings ...
Lastly, do his pax try and bring too much with them, risking overweight departure? Especially if he has to tanker fuel for his get out, which would be to a third point, not his original.