How long would it take to make a 4K remake of Ultimate Doom, no i dont mean the type of remake ID Software is doing, i mean a remake Of Ultimate Doom, like the level designs are the same, weapons and textures the same but with 4k textures, sounds and music are same but is not the same 16 bit midi music but rather a 32 bit mp3 the only big change is graphis are so hd, it sort of would look like a new game but in the end it isnt
Depends on how good / fast you are 
The hardest trick would be to get the lighting right.
The lighting of UE4 is just too good 
What if the person coding it was someone like John Carmack or John Romero?
I don’t quite understand??
The lighting in Doom is sector based. You have hard contrast changes in the lighting from one sector to the next. there is no smooth falloff of light sources.
You would also need to make a lot of your materials dimmly emissive…
It would take as long as any other game minus the planning and testing gameplay philosophies.
Ahok, yeah i forgot that Doom’s levels were sector based, heres another question why did sector based level design die.
It’s still being used but the transition between sectors has become seamless.