What portion of an Instrument Proficiency Check must be in the airplane?
This is explained in the Instrument Rating ACS (page A-18):


  Instrument Proficiency Check  
  
  If a person fails to meet the experience requirements of section 61.57(c), a pilot may only
  establish instrument currency through an instrument proficiency check
  as described in section 61.57(d). An FSTD may be used as part of an
  approved curriculum to accomplish all or portions of this check. If
  specified in its LOA, an AATD may be used to complete most of the
  required Tasks. However, the circling approach, the landing Task, and
  the multiengine airplane Tasks must be accomplished in an aircraft or
  FFS (Level B, C, or D).A BATD cannot be used for an instrument
  proficiency check. Please see the Task Table in Appendix 5 for
  additional information. (Appendix 5, Instrument Proficiency Check
  Table).


To summarize:


If the simulator is an approved FFS, you can do the whole IPC in it
If the simulator is an approved AATD, you can do everything except circling, landing and multi-engine tasks
Otherwise, you need to use an aircraft