You know, if they really want to be like Frontier -- it's not too late to license the Cobra engine and just port the assets over. It's not like there's much useful code there to salvage, the only good stuff so far are the assets.
If this project is to fully embrace it's destiny as a modern Duke Nukem Forever, it needs one engine change at LEAST in it's troubled production history. Forget UE4... forget CryEngine 5... Hail Cobra!
I'd actually be curious to see how their assets are rendered in Cobra. I don't think the lighting and shaders would be as good as they are in CryEngine but Cobra is clearly a lot more scalable.