Not really that much, it's the same code for both modes, everything is duplicated in memory, the only thing that changes is that the remastered objects are drawn in one rendering pass and the original objects in another pass, and the switch between the two is done with a blend. It was mainly me who devoted myself to making the original rendering as close as possible to the original game and I was happy to do it
For example, at one point during development, the plan was to have the original rendering with a disgusting pixelated filter, so that the switch would be highlighted. I complained so that we wouldn't do that.
Instead, I made sure to have the same draw distance for the original mode as the original game (I imported the values from Kal). And that convinced them.
Besides, in the remaster, there are a lot of visual bugs because the draw distance is stupidly too far
