The general shape of the model could be based on this Lockheed proposal from the early fifties(document Lockhed)
The Lockheed Weapon System 125 of 1955 has the needle-thin fuselage , the high horizontal tail and the straight wings of the model buut the engines are grouped close to the fuselage. (reserved rights)
This small picture from the Life Magazine illustrates straight wings and engines well off the fuselage but strangely with the reactor still inside the fuselage . The tail is obviously a carry over from wartime bombers which would be an impractical configuration for such an aircraft (collection A.P. Yanchus)
The CL-400 , a design preceding the SR-71 was not nuclear-powered but hydrogen powered . Still its design is quite similar to the Hawk model . And would have been classified at the time !(document Lockheed)
This Martin design could have provided inspiration for Hawk with its wingtip mounted jets and parasite fighter (or is it donkey aircraft) Yet all these designs were probably classified when the model was designed ! (document Martin)