My server crashed during update, how to handle svn repositories on new installation?

I have an Ubuntu Server 16.04.2 LTS that is running Apache Subversion
1.9.7 (r1800392). Total size of repositories is 5.5 GB. It was
installed a year ago approximately.
This server is acting as a backup to the main repository using
svnsync.

Today I was going to upgrade the Ubuntu server to version 18.04.2 as
suggested when I logged in to the server (using SSH). So I was going
to use the week-end to upgrade.

But it went south on me, the do-release-upgrade process chugged along
until it suddenly disappeared from my monitor. Turns out something
went really bad with the upgrade, but all data files seem to be OK.
Except when booting the server goes into PANIC mode.

So I figured I would have to extract the disk data to an USB disk or
similar, before installing Ubuntu Server 16.04.2LTS from scratch
again.
In the console which I can reach using an earlier kernel I can tar up
the SVN repositories, so if I can just move them out of the PC as tgz
files I would be OK.

Once that has been done and Ubuntu is reinstalled I need to attach the
files from the original disk to the svn server. How would that be best
done?

If I extract the archives into /var/lib/svn would svn be OK with the
files then?
Does it do some magic like marking the files in any way that will make
the sync from the main server fail?
Or is the ID of a repository stored in some of the files inside the
repo database?
The svnsync function relies in some way on the repository ID,
right....

-- 
Bo Berglund
Developer in Sweden