Just a point to note out.
When you find a video that has been deleted in your Favorites list on Youtube, you can take the url for it and put it into a google search. Google typically shows you what the name of the url had before it got deleted.
↑ ↑ ↑ That might also work for our broken urls, I think...
If all else fails, Waybackmachine may have saved the forums way back in the day, it is usually pretty limited though. 
EDIT: It seems as though Waybackmachine mostly just has the top level of the forum. It is possible it might have a particular thread saved in it, but I would say that it is unlikely.
Still fun to see what the forum used to look like though.