I already answered on another topic.
Hellkeeper wrote on Nov 3
rd, 2020 at 6:06pm:
Your polygons are flickering because your entire map is one single gigantic brush with no inside and no outside. I'm actually surprised UnrealEd manages to display anything and imported that without crashing.
How to fix that: easy, rebuild the entire thing. I guess it's a full N64 level from Ocarina Of Time imported and subtracted. It doesn't work at all (it's not even the right scale, the sword itself is higher than a player). You can keep your... Thing, right now, as a plan, then make normal brushes to match and delete the original after you've completed the clean version. I had done such a thing with parts of levels imported from UT2004 to Unreal or the other way around, though in your case, you have to rebuild everything, not just a part of it.
IN SHORT: Your map was made for a completely different engine and so its geometry doesn't work correctly with Unreal. Stop trying to import the map and BUILD the map. It's not hard, it's a map made for the N64, it's super basic. Just launch your editor, import your brush and use it as a floorplan to rebuild the entire thing, YOU CANNOT DO ANYTHING WITH THIS IMPORTED GEOMETRY and you either need to use another method to convert it to something Unreal can use or make a script somehow to process this imported geometry into something that can work.
There is
nothing you can do in UnrealEd to make this functioning in a few clics, it's not UnrealEd or Unreal's fault, there is no magic anything to have this work smoothly. You just need to use the editor for its original purpose and REBUILD the map, using your imported spew as guidelines that you will delete once you're done rebuilding everything properly and cleanly with the normal tools at your disposal: time, brushes, patience.