Well if as you say the ODST helmet is a slightly mis-shapen Spartan helmet couldnt you use the Pepakura MC helmet and just modify it? or is that not viable?
well to make an ODST with the MJOLNIR helmet start you would need to clip out the visor, and lengthen the face plate, remove the flat surfaces for light's on each side and blam ODST. Feet first into hell.