I made a few attempts to repair the model with my "usual" tools, but the real fix lies in using Blender. I attempted to learn blender a few years ago, but let my skills lapse to the point where I'd have to go through the tutorials again to be effective for this model.

I was able to properly select the edges and vertices and even able to tug them in the right direction to close up the holes, but my lack of skills frustrated me to the point of surrender.

I also wonder if my equally poor skills in Fusion 360 would be an answer, using the Sculpt mode to close up the holes.

The wing openings are the big problem, along with the area where nose gear would appear, but there appears to be some zero thickness surfaces here and there.

We occasionally have a member to our makerspace who professes to some skill in Blender. I'll drop the file on my laptop and when/if he appears, have him take a gander.