The closest single-pattern Schoeps capsule to the omni setting of the MK 5 is the probably MK 2 S. The equivalence isn't exact, though. If you're very attached to that exact sound, I'd urge you to hold out for the real thing; a good used pair should come along sooner or later.
That said, the effective high frequency response of omni microphones--what you actually hear in the recordings you make with them--is the most sensitive of all patterns to variations in miking distance. In any given indoor recording space, there's always a lot of interaction between { the "spec sheet" high frequency response of the microphones } and { the room and the miking distance } as they affect the sound pickup.
--best regards