I am actually modifying a package under the GPLv3 License which according to a page requires to:
Include a copy of the license and copyright notice with the code.
However such package/code don't have the license nor the copyright notice on each file. Can I modify significantly the code of such package?
If you know that the code is GPLv3, then you should also know that you are not allowed to file the serial numbers off of it and slap your own copyright and license on it.
If the original code is GPLv3, your modified version of that code must also be GPLv3.
If you're so inclined, you can contact the copyright holder of the code and ask them to remedy the deficiencies you have noted by adding the necessary copyright statements and license files.
