Legally, yes, you can, since the source code is freely available, and you (as a rule) have allowed it to be redistributed. Is it kosher? I think it depends. Many people will reflect repos on GitHub, especially if the canonical repo is only available as an SVN (or other) repo; in this case, the person who places it on GitHub will usually have the same name, with a note that this is a mirror of the official repo.
If you not only mirror, but also deploy the project and post it on GitHub, perhaps you should change the name of your fork (if you do not intend to fully expect your changes to be merged into the official repo for a later date) .
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