Re: A venerable workhorse
@bazza, it's not a problem... Remember that most of these jets are the old -100 or -200 models, which were cheap to get (or were donated by their old owners, in the case of SOFIA). There is a plethora of -400 series jet available given that Cathay and Air France have recently retired their last -400 bodies, and British Airways still has over 30 of them in daily use.
GE and Rolls Royce (and NASA) will have *plenty* of jets to borrow/buy/gut/Frankenjet. The Airbus A340 is also still a usable airframe.
Air Force 1 (actually, you mean the VC-25A, the military designation of the B747) is not a problem either because there's an existing requirement from the USAF to Boeing to produce a copy or three as a replacement for the existing airframe. It will likely be a -8 model.