Lost my first logbook when my car was stolen. Went to the FAA and got a copy of my previous 8710s. I then took these 8710s to show proof and included a document in my next logbook that said that the flight time forwarded into the new book was correct and legal with a signature. The FAA took this to be good enough for me when i took a 135 check ride. The big thing was the signature sheet that stated that the time was correct to the best of my knowledge. Hope this helps. Since then I keep all my time in excel as a backup.