^^ A sense of credibility was not this design's forte. The only visible means of propulsion originally was the collar of rocket tubes encircling the centre section. Those tubes look awfully small and when you consider the forward section was, as you said, essentially hollow then just where were the motors for those rocket tubes? Now one little tidbit: in the film there is reference to a "gravity neutralizer" and in the serials it's called a "counter-magnet." Both terms suggest the idea of an antigrav system in play. In that case then the rocket motors mightn't need to be that big and powerful to get the ship off the ground and for maneuvering since the antigrav system could well be the main drive. The collar of tubes could essentially be maneuvering thrusters.
In the film Just Imagine
the four rear facing openings (or at least the upper two) were indeed windows, but during the Flash Gordon
serials they were turned into rocket exhausts.
Now if you go by the full-size exterior mock-up it does indeed look like there is next to no room for mechanicals. But if you go by the filming miniature (which has a proportionately smaller access hatch) and the interior set shown in the first serial then you get an impression that the ship is somewhat larger and the is indeed room for mechanicals. Indeed the shape of the forward compartment seen in the first serial doesn't match the shape of the exterior forward section. The interior seems to have flatter walls that don't match the curved shape of the exterior.
With that perspective I have been able to fashion a rudimentary interior that does allow for quite a bit of space for mechanicals between hulls with most of them under the deck/floor. Now you can't really make an exec match of any of the interiors (save the film version) that properly matches the ship's exterior shape. To that end the best I can is use the basic template of the interior seen in the first serial and adapt it to make a more credible and consistent (with the exterior) interior.