Depends on your setup... Maybe you can simply dump your filesystem.
You need to Backup mysql, mail, web, /usr/local/ispconfig, some files/dirs in /etc and so on.
I backup each night nearly the complete systeme (excluding web and mail since it´s already replicated to other servers). If one server crashes, i can install ISPConfig, re-import all databases and copy all files to the fresh installed system.
But I`ve also an image for the server available by ftp to use it during an fresh install. Since most config-files are changed rarely, it should work out-of-the box as long as the image is not to old. Such an image could also be restored in runlevel 1 (or in the rescue-mode) by simple extracting the archive.