file: /// most likely does not work because your SVN repository is in FS format, newer than the one that Git knows about.
Your second approach with svn: // localhost was in the right direction. However, you need to host the repository using the svnserve command-line utility. Sort of:
svnserve -d -R --root file://path/to/repo
After receiving svnserve for the correct repo placement, you can use the path starting with: svn://localhost
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