Moving to multiplayer would require a server that can run alongside the instance of the game.
There may be two possibilities:
Implement a node server that runs alongside ImpactJS and figure out how to deploy publicly
Migrate codebase to a custom game server (not based on ImpactJS; as it requires the window variable to exist) and create browser-facing clients that send input data and receive game updates from the server
Moving to multiplayer would require a server that can run alongside the instance of the game.
There may be two possibilities: