Is it not possible to automation android app using espresso without source code. Gradle expects a structure like this:
src/main/
src/androidTest/
as i understand older version of your app doesn't have espresso tests.
Open a commit with different version of app, into androidTest include tests and run assembleAndroidTest Gradle task to make an app which already would have included your tests.
This version of app might be useful for Testdroid platform.
I'm not an expert in this, but I can also suggest you to use monkey tool
Read about it here: http://developer.android.com/tools/help/monkey.html
and if you know a bit Python you can use monkeyrunner for this.
Check: http://developer.android.com/tools/help/monkeyrunner_concepts.html
These two tools will help you a lot to run the test on the different versions of app.
Hope it help