"MUST"? "equivalent code"?
It is a wrong idea. Game mechanic is well know, that's why the java emulator was made, some guys try even CSharp or javascript. Nothing prevents you from writing such a new game engine, modern, based on newest tech stack and modern compilers. How many players can hold l2off Interlude? 10k? 15K? How about 100k? 200k? How can I be sure that l2off project was perfect? The way @MasterToma is the dead end. Technology and ideas from 2000, not 2020.
The problem is here - developers community. Try look at WoW emulators - C++, good documentation, opensourced.
In world of L2 dev-admins - money, greed, closed, bad code, lack of no documentation at all.
And at least but not last - best game engines (UE4, Unity) are now available from zero cost at beggining.
(Again, LU4 closed project and not finallized, maybe 1/6 or less content after 5 years and only client side)
Many free opensource tools - Blender , free assets etc
Theoritically nothing stops You from making a modern opensource AAA grade game . Only lack of skills and time.
Wake up ppl, we are living in 2020