I don't know if I'd say the bulk is done, but it definitely could be done if necessary. It might not be a bad idea anyway as there are several architectural issues that could be solved in the process.
The biggest chunk that only exists serverside, besides the volatile data (characters, etc) of course, is the mission content. Much of that is documented on the Paragon Wiki and could be reconstructed. It wouldn't be perfect, but we could get fairly close.
There's a few other odds and ends that don't exist clientside, like AI pathing beacons, but a lot of that is implementation-specific and wouldn't necessarily apply to a rewritten server.
The biggest issue that would have to be faced in that circumstance is that without source code for the client, any substantial improvements can't really happen until the client is rewritten as well, including all of the graphics code.
