Not weighing in on this specific rewrite but tests are how you specify that your software works correctly. If a behavior isn’t covered by an automated test in some form you can’t assert that any given change doesn’t break it.

I think it is completely reasonable to use a preexisting unmodified test suite to state that something is working. The larger the project the more true this becomes. Real world production scars are documented and guarded against in the test suite otherwise those lessons get lost.

Also SQLite is legendary for its massive test suite and extensive fuzzing. They have 590x the amount of test code and scripts than normal code. Source: https://sqlite.org/testing.html