If you are specifically looking for a private server experience for the original Sudden Attack (not SA2), some community-run projects still exist. Sudden Attack Zero Point on Steam
Sudden Attack 2 was famously shut down in 2016 only months after its release, but a new private server project has recently launched to bring the game back to life. sudden attack 2 private server link
Disclaimer: Private servers are not affiliated with Nexon. Use them at your own risk. Always run unknown executables in a sandbox or virtual machine. If you are specifically looking for a private