On Debian-based distros the easiest solution is:

apt-key adv --recv-keys --keyserver keys.gnupg.net 63F7D4AFF6D61D45

I found that Opera 12 conflicts with Opera Stable, but installing Opera Next is possible and it is the same build 12.16.1860. It uses ~/.opera-next as the configuration directory, so you might want to create a symbolic link at ~/.opera if copying an existing profile.