So, I haven't been a part of the networking team on any of my prior games, but I have worked on 3 separate networked multiplayer games in the past (2 in Unity, 1 in Source,) and from what I understand, the most difficult part of making a singleplayer game like this into a properly realized multiplayer game would be, firstly, setting up unique game objects to recognize multiple players, and secondly, figuring out everything that would need to networked across players. Anything that acknowledges the player in singleplayer would need to be able to differentiate between players in multiplayer. Anything that can't be implicitly synced would need to be networked. There'd also need to be a functional networking backend server unless you'd just want to commit everything to the clients by making it peer-to-peer. Not to be discouraging, of course, but I wouldn't hold my breathe.