JammedStab
Engine internals rotate in opposite directions to reduce the torque loading on the engine mount system as noted above.
The actual differences between the engines can only be determined by the changes in parts as per the Illustrated Parts Catalog and the Service Bulletin status.
In general the -20 was a baseline engine - and was developed for both Beech and DHC.
The -27 was an upgrade to the hot section and used the new integrated governor system that did away with the "Zed" Gap governor system.
The - 28 was primarily developed for a high altitude application - and was used in the Beech King Air and the Piper Cheyenne 2 for example.
Further development of the -34 was to increase power and reliability - used in EMB 110 and in late model DHC-6.
I have added another file to the lot available by the link above. It is a simple table with the engine differences.
Hope this helps.