Start a new save and import the Gen 5 Living Dex pcdata.bin from here. It has all Pokemon but on different levels.
Alternatively you could use the ALM plugin and generate your own Living Dex mons. All will be level 100.
https://github.com/santacrab2/PKHeX-Plugins/releases/latest
The file is full of garbage, no real save data inside.
Besides that the proper size for Gen 3 saves is 128kb. You should check if FlashGBX is able to dump the full 128kb save.