To enable it, go to settings.yml -> Options.WeaponMechanics and set it to true.
Usage example (kits.yml):
ITEM : AMOUNT : weapon : name:Weapon Name
You can put any item here, it will be replaced to the weapon.
I'm not sure if using special characters on kit name at the top (AK47) will cause any problems, avoid it just in case.
Result:
Now, note that the weapon will take priority over the lore from Wizards, if you want to put a custom lore with a price on it you cannot use the Weapon on the display-item setting of the kit.