I’ve had the head gasket professionally done on my truck by a friend (under 100k at the time) a few years ago. The head was removed and decked. I had timing belt and water pump changed (might as well while the shit all apart) when it was done. I’ve got a great local mechanic friend here in Brooklyn who did the job for me.
Not everyone thinks getting the heads decked needs to be done and I know a crazy Russian guy that successfully did his own without.
Is your dad's truck over heating?
I also know a guy in PA that swapped the engine on his 1st gen and could ask where he sourced the part and who did the work if ya want.
I’ve got a lot of mods and personal love in my truck and have always thought of running it until one of us is in the ground…
That said; if I did not have any mods and/or did not wheel and/or have my wife’s CX9 to drive when necessary I think I’d move on the new vehicle rather than fix or swap the engine on a 2001 Xterra with 206k unless I wanted to do it myself as a project and did not depend on the vehicle.