Not all is entirely lost.
You can still use google search to search the old forum:
In a google box, type "site:forum.concours.org {search key}"
For example, to find posts about kipass:
site:forum.concours.org kipass
You can also use the "Show search tools" in the left margin to limit the date range.
This is not an ideal solution, but at least gives access to some of the old content... at least until google purges it.