1. Open a File Explorer window and type %localappdata% in the address bar
2. Open the folder SamuraiGunn2
3. Open the file questdata1.sgq in a text editor
4. Find the part of the file that reads something like "weapons":{"0":1}
5. Change the part found in step 4 to read "weapons":{"0":1,"1":1,"2":1,"3":1}
6. Save and close the file questdata1.sgq

If you do not have a save file to edit for whatever reason, simply open a text editor and paste the following, {"weapons":{"0":1,"1":1,"2":1,"3":1}} then save as questdata1.sgq in the location mentioned.

Note that you need to load in to adventure mode every time you start the game in order for your save and weapons to be recognized in versus mode.

Have fun!