Adding on to Rich's answer, which I agree with 100%, it also depends on what you mean by modify. As in replace with a water-cooled system? You'll need to know how much vertical clearance you'll need to install something like that. There's quite a bit of space above the CPU, the outer case is 8.25" wide outer dimensions, but better not to guess and hope.
And I'm also wondering why? The CPU can't be overclocked, so why mess with the factory cooling system?