Scrap Mechanic save is a SQL Lite data base. So you need to download SQL Editor first. You can use comand prompt sql editor (sqllite3) but you need to learn sql language for it. I recomend to use GUI database editors like SQL Lite Studio[sqlitestudio.pl] or other.

REMOVE FANT FILE MOD!!!

Open your saves folder.
For Windows: AppData/Roaming/Axolot Games/Scrap Mechanic/User/User<id>/Save/
For Linux: steamapps/compatdata/387990/pfx/drive_c/users/steamuser/AppData/Roaming/Axolot Games/Scrap Mechanic/User/User<id>/Save

Copy your old save
From "Survival" folder to custom folder copy file <your_save_name>.db

Download fant custom game mode
Link: https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2820096455

Make new raw fant custom game mode file
Scrap Mechanic -> Custom Game -> New Game -> Fant Game Mode -> <fant_save>.db
Exit game.

MAKE BACK UP OF YOUR OLD SAVE
Copy <your_save_name>.db to folder where this file will be in safe.

Edit your old file with your SQL Editor
Open <your_save_name>.db and <fant_save>.db (locates in Custom folder)

Find table named <Game> in <fant_save>.db:
<Game> has next rows:
--- savegameversion
--- flags
--- seed
--- gametick
--- mods
--- uniqueIds

In <your_save_name>.db table <Game> has next rows:
--- savegameversion
--- flags
--- seed
--- gametick
--- mods

You need to create new row <uniqueIds> in <your_save_name>.db
Then you need to copy values from rows <mods> and <uniqueIds>
from <fant_save>.db to <your_save_name>.db
After that you need to save changes to <your_save_name>.db and close the SQL Editor

Enjoy your game
Open Scrap Mechanic.
Load your "old" save.
Wait until game figures out "What the heck is wrong with this save"
Play your ported game.

Delete raw game save <fant_save>.db
You copied <uniqueIds> from it. So now we have to games with same <uniqueIds> field...
So it's better to remove <uniqueIds> donor from the saves folder to exclude unpredictable game behaviour.


If everything works fine you can delete SQL Editor if you don't need it.