I have no issues navigating around in an old rails 1.2 app. The problem is usually the special sauce and anti-patterns that have been added to it over the years. This is a problem regardless of language or framework.
Running a Rails 1.2 app sounds... dangerous. Have you plans on upgrading it? (Probably at this point easier to start a fresh Rails 5 app, write some tests + copy/edit stuff over as needed).