I too have seen the claims that "all that was necessary" was to turn the crankshaft and camshafts end-for-end (etc.) without explaining how and why that could have the desired effect.
These might help:
Allison V-1710 Gear Train for Right-Hand Rotating Engines
Allison V-1710 Gear Train for Left-Hand Rotating Engines
AFAIK, engaging the extra idler gear on the LH rotating engine reversed the camshaft rotation (no need to flip it), though obviously the crankshaft had to be reversed and the magneto wiring changed as previously mentioned.