They definitely don´t use the 767 nose cone, as this isn´t approved by the CAA ant the FAA.
The only reason is, that this is simply a spare part, perhaps from the original manufacturer or from a spare part company or even from a different airline ( I guess World!). They´re usually painted in white.
Sure, retro jets for each of the majors would look nice. We' all love to LOOK at them and PHOTOGRAPH them. But would that generate any new traffic? How many of you would actually FLY on it? Would the travlling public as a whole be wooed by that kind of program to warrant the cost of a one-off paint scheme?
I highly doubt it. And that's why you don't see more retro jets and why it's unlikely you'll see many more.