The F3 functionality was removed via this bugzilla report.
The bugzilla report contains the patch used to actually remove the functionality.
As you can see from the stats - it was a sizable patch crossing many source files
19 files changed, 916 insertions(+), 1847 deletions
Thus, to reinstate this, you'll need to reverse engineer this. What makes this more difficult, is that the code base of nautilus has changed considerably between Nautilus from Gnome 3.2 when F3 was available to todays Gnome 3.10/3.12 code base.
Thus its not going to be an easy, "take this diff and insert into the source file". You'll likely to have to manually match up the various functions and manually edit the source.
To build nautilus is relatively straightforward:
apt-get source nautilus
sudo apt-get build-dep nautilus
Make your changes. Compile via either running the autogen in the top-tree level or seeing if a
debuild -us -uc method works if you've patched the source files in the
More information here how to build.
If this seems like hardwork - it is. Good luck. I'm sure you'll have a grateful community if you manage to hack it.
If you like an easier life, this Q&A gives alternative methods and alternative file-managers that has this capability.