How to recover a very old repository?

[Dale Mellor]

I've recently pulled an old SVN repository off a disk image from a
server I had many moons ago, and now have the repository on a local file
system.  Whenever I try to do anything, SVN complains that it is format
'2' instead of '3' or '5'.  What can I do to recover a working tree of
whatever is in there?