My search string was FF FF 00 01 00 for the game v1.1.36.

1. Unpack your save zip (on my PC it is located in c:\Users\keht\AppData\Roaming\Factorio\saves\). Make sure you have a backup!
2. Find a lot of 'level.datX' files
3. Starting from the last file do:
- Using the tool provided by pawelzwronek ...