I hope it's ok if I plug my own little "Spacewar!" hobby project. It's not as true to the original, but it automatically matches people connecting to the game to have them play against each other:
Totally ok :-) I was always thinking of doing this with the original game via WebRTC DataChannels (that's also a reason for the emulation running the display asynchronously, so one could be driving the other).
BTW: By this Spacewar! would then become Orbitwar (Silas Warner, 1974 / PATO network).
http://orbital.lewix.eu/