Nothing about Starfield is that amazing that you couldn’t replicate it in something like Unreal or even Unity.
Graphics are dead easy on either. Exploration is faked, it’s fast travel to a procedural terrain/level, with a few hand made destinations in between, nothing hard. Modular ship design? Simple. FPS RPG system, simple. Physics engines already exist, storing the location of player placed objects is trivial.
What exactly about Starfield makes you think an engine would need serious modifications for a SF-like game?