Thousands and thousands of hours is more appropriate
I don't know how you could change the engine without rewriting the entire thing basically from scratch.
It really depends on how modular their codebase is. The Doom 1/2 modern ports they did in 2019 use Unity. But it's actually still the original Doom underneath and just using Unity for input and output to make porting easier
Many many hours is a massive understatement.
Thousands and thousands of hours is more appropriate
I don't know how you could change the engine without rewriting the entire thing basically from scratch.
It really depends on how modular their codebase is. The Doom 1/2 modern ports they did in 2019 use Unity. But it's actually still the original Doom underneath and just using Unity for input and output to make porting easier