You can just add a second line to /etc/passwd with a different username but the same numerical uid. Like this:

    altroot:x:0:0:Alternative Root User:/:/bin/sh

We used to do this all the time for users who needed root access to their own workstation. It allowed us to avoid telling them the common root password used on all the machines in the organization.

In your case, doing this might be beneficial in case there is a network problem because you'll have a way to log in as root locally.

Back in the day we would've just added our IP to the .rhosts file and no password would be required at all!

It does have me thinking about what versions of SSH would run on such an old OS. I'm sure there were versions available at one time... and since it's vulnerable to remote exploit anyways the version wouldn't really matter.