Great project !
On top of Empire State Building… you’d probably reconstruct Empire State Building as well… but putting King Kong on top of it should not be a problem 
set the full moon with FS and include a King Kong mesh on the correct altitude, there are many, there could be a .GLB
https://www.google.com/search?q=mesh+3d+king+kong&client=firefox-b-d&tbm=isch&source=iu&ictx=1
I think it could be complicated to place King Kong accurately on top, when you do not create Empire State Building itself… ground level may be slightly different from Twin Towers… wonder if actual altitude of the Empire State Building (GL+381m) could be available in the MSFS files… I found this on the Cesium website,
https://demos.cesium.com/NewYork
If you mouse over the city and click buildings, you see indexes, like Bldg_12210009196, there may be a BLG file somewhere ?