Having the source code does not really have any bearing on how much work it is to re-draw the game's graphics. It might make it a little easier to tweak the game's graphics engine, but it's not a magic bullet where you change a number and suddenly the game knows how to draw more pixels, and anyway many of the graphics subsystems have already been tweaked or rewritten in JJ2+. Even if the source code were available, it's still the source code of a game that was designed to never run at resolutions higher than 640x480, and changing that would still be a lot of work.

As for why Epic Games does not seem to be interested in open-sourcing the game, your guess is as good as mine, but the fact of the matter is that they aren't and that is very unlikely to change.