About the engine: the engine is the original Halo Combat Evolved or Halo PC for the CAMPAIGN (e.g Jumping, throwing nades, meele), they just made a new graphics engine (wich can be disabled with just pressing the back button and you will see the original halo graphics) both graphic engines are running at the same time.
Multiplayer is the ONLY one using reach engine, the maps are for reach multiplayer, they can´t be played in the ORIGINAL engine.