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.

^{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.