@japaokiko
You can download the sandbox server, the code is not pretty, but it's enough to walk around in the world, even with other players at the same time.
It's just missing all AI/combat related stuff as that's way harder to reconstruct.
To add that, you most likely don't need to mess with the geometry files (exception may be the terrain for pathfinding and LOS checks, but those are the last things to worry about).
For everything else you could have a look at the current state of the package extractor we have assembled so far.
You can find that on github (i'd suggest also looking at https://github.com/TCoS-Rebirth/PackageImporter for an improved version)