You can run whatever client you want.
> If I own the mechanism of root node discovery, I can mount an effective Sybil attack against a new network participant.
But you wouldn't be able to replicate a chain of blocks with the same order of magnitude of work behind them. And then, even if you could, all you'd be able to do is change history, with the depth of that history dependent upon how much mining power you had available. You wouldn't be able to spend other people's money or give yourself more.
