Tamiya makes a 1/100 Space Shuttle and I've seen a few models where it was combined with a Nitto/Entex/Doyusha (all the same mold) 1/100 747-100/200 to do 747 carrier aircraft with shuttles on their back. The struts naturally and extra vertical fins on the stabs have to be scratchbuilt, but it is indeed doable. Doing a real shuttle naturally involves having to make a tailcone for the orbiter. But, Moonraker doesn't have those concerns.
I won an Entex 1:100 Boeing 747-100 (1:100 scale) model on ebay. This plus the Tamiya model kit makes the Moonraker-747 model a possibility for me. There is a Canadian who has 1:100 American Airlines 747 and 1:100 Moonraker decals for sale so this project becomes very doable.