Can a GPS system be used to identify fixes on conventional instrument approaches?
Your reading of AC90-108 is correct. As long as the RNAV equipment is operated and installed in accordance with TSO-C129/-C145/-C146 it can be used as a "suitable RNAV system." (ref: AC90-108, para. 6 a.).

Also, see the AIM, 1-2-3 b. "Types of RNAV Systems that Qualify as a Suitable RNAV System."

Lastly, as far as the requirement for "Dual VOR Receivers" for IZKIP, since you are using a "suitable" (and legal) alternative method by using the GPS database fix location, no additional restriction should apply. Also, it is my understanding that when the subject stepdown fix (identified by a VOR radial) is inside the FAF, as is the case with IZKIP, the dual VOR requirement is for the purpose of presetting the missed approach holding fix.  Again, not an issue if you are using a "suitable RNAV" system for identifying the fixes.