Your only option besides starting over is to mod in the console and type the code for it. If it is available via console. Here is one persons instructions. Though I'm not sure if it gives the unique one so you can then craft.
1. Download the command center
https://www.nexusmods.com/cyberpunk2077/mods/147
2. Unpack the Zip file and open the instructions. Follow every step there.
Also more precise drag the DLL files into Cyberpunk 2077\bin\x64\
You can find the folder on gog launcher
3. Open the game and press Tilda ~ or ` when ingame. The button under ESC. If you are not using QWERTY keyboard layout, this won't work. Change your keyboard layout on Windows language settings.
4. Type
player.inventory.addItem(Items.Preset_Katana_GoG,1)
Make sure you typed every letter like I did. with the same caps. And enter
5. Enjoy the black unicorn. You should have it back. You want more of these just change the last number in the command to the amount you want.