Depending on the configuration of the Trac server, you may find the "zip" link at the bottom of the source browser for the trunk directory and subdirectories of branches and tags. This will give you a zip archive of this tree in this version.
I know that I saw someone writing a tool to download the source tree from the Trac browser, but I can not find it. Perhaps this was an entry on the trac-dev mailing list.
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