Codebase looks old and unmaintained.
Technically you could fork it and make changes. However due to the lack of a open source license the code is automatically propitiatory and if the original author got wind of it and want to bring down your project due to copyright, he/she could.
Practically speaking it’s a bit of a gray area. I’d say do whatever (since it’s open source), but respect the original author’s wishes if he ever contacted you. In my opinion, if you were to host and maintain your own version/fork of this, give the author credit where due and don’t try to monetize it.
As for me personally, I wouldn’t touch that code myself. Mostly due to a lack of proper framework and possible security issues. I’d use my framework of choice (Laravel) and build my own version from scratch. But that’s just me.