Old Game Graphics Update (Shooter game)

We have an old 2003 game called GunZ: The Duel (Third Sport Shooter Online), we want to update the graphics of this game to something more modern, including the graphics of the game and the interface of the game, but with all these updates, the game itself should remain the same as the old. For this task we will make available the source code of the game. And the game files to be edited. We uploaded a picture of what the game looks like at moment.