Android - Is there are way to change Android's default screen orientation?

Run this from adb shell or from a root terminal:

content insert --uri content://settings/system --bind name:s:user_rotation --bind value:i:0

Change the value to 1 if you want to go back to Landscape (2 and 3 also work for upside-down portrait and reverse landscape).

There's an app called Set Orientation which looks like it does what you need:

Set Orientation allows you to force your device into a particular screen orientation.

Set Orientation Set Orientation
Screenshots of Set Orientation (source: Google Play; click images for larger variants)

As the screenshots show, this app let's you override the sensors (which you don't have) and tell the system which way your screen is oriented.

Update: the app was unlisted from Play, but you can find some alternatives here.