Good luck finding S-N curves for any zinc alloys. This is only one of many areas that makes "engineering" zinc components problematic. The best that I can do is provide some numbers from the only decent publication on zinc alloys that I have found-- Engineering Properties of Zinc Alloys, Third Edition - Revised:
ZA-27 die-cast - 145 MPa (21 ksi) at 5 x 108 cycles
ZA-27 sand-cast (as-cast) - 172 MPa (25 ksi) at 5 x 108 cycles
ZA-27 sand-cast (heat-treated) - 103 MPa (15 ksi) at 5 x 108 cycles
Fatigue data was obtained from room temperature tests conducted under rotating bending according to ASTM E 446. Machine speed was 10,000 rpm.