I don't care about 3.1, you are the one saying "3.1 this, 3.1 that". I skipped 3.1 altogether, and moved on from 3.0 to 3.5 in 1999 once I got my prerelease of 3.5, and would probably have left the platform at that time (as far as writing new software for it) otherwise. 3.1 is just too lacking.
If you want something "modern and complete but still compatible" you will be pleased with 3.2. Unfortunately AROS is neither modern nor complete nor compatible, and would not be even with open access to 3.1 sources (since 3.1 itself is none of those things).