Is there an open-source HN forum clone?

FWIW some years ago Salvatore Sanfilippo (antirez) wrote Lamernews [0][1] with the goal to create such a clone. AFAIK Lamernews, as a site, is long gone, but the code -Ruby, Sinatra, Redis- is still there.

There have also been some forks. EchoJS [2] made a few modifications through the years and is running still today. Mostly about moderation and some related tasks like blacklisting and others. Can't say much about pros/cons but at least it's still used and somewhat maintained.

[0] https://news.ycombinator.com/item?id=3146051

[1] https://github.com/antirez/lamernews

[2] https://github.com/echojs/echojs

You might find something useful here:

https://github.com/arclanguage/anarki/tree/master/apps/news