Is it possible to clone a site from this page, modify it, and use it in a client project?
Most likely. You can still contact the author to get his approval.
As vincent pointed out, while a project may be clone-able, it is always best to contact the author and establish whether you can use their design or assets. A lot of designers share their work for learning purpose, and may also have used material that they do not have rights to (hence why you will see a disclaimer on copies such as the apple website ones).
When in doubt, always best to ask