How to continue to run 'git svn fetch' for a repo with no git-svn metadata

I cloned a repo from a central repo which is mirrored by git-svn. Now
i want to mirror the svn in the cloned local repo by myself, however,
i don't have the git-svn metadata (which is in the central repo). What
should i do to run "git svn fetch" without refetching all the
versions?