Rework are necessary. Except if you're happy with custom gameplay.
Let's take skills for instance :
Targeting of L2J is based on a single type, where retail has 3 (more if you count fanrange, affect range, affect limit etc), you'll never achieve the same
Formulas are so fucked up ...
Stats are fucked as well, critical for instance is not properly calculated on L2J
If a skill have a single level, e.g 3, you need to create level 1 to 3 on L2J
Same for effect, you can't attach any effect to a specific skill level
I could go one for a while, better stop now, but you see the point