In most cases private servers are created from scratch with no source of a game at all. It takes a lot of work to build a private server. Things such as:
- Coding experience to even get started on the backend of the server.
- File analysis of the game data to use with the server.
- Packet analysis to understand the communication layer between the game client and your server.
- Packet encryption (if any) that is used to protect the data.
And so on. It's not an overnight thing to make a private server, it is a very long process to get something started and working.