Unity3D VR: Add multiplayer mode to a finished and working game

Integrate multi-player mode on a Unity 3D game using a VR headset. The game is already over. It works with Oculus VR glasses. 2 players must face off in a duel by throwing fireballs at a target and the first player to score 33 points will win the game. We are going to set up a metamask wallet for players to place bets on each other.
The game should be compiled for WebXR / VR
Thank you