I think the first option would be to peen the other side, but the other side was masked off for a reason. Even if allowed, the part may not return to its original shape.
More important is learning why it warped. Was it peened with too high of an intensity? If so, that would be the larger issue and cannot be undone. If the peening procedure allowed for the intensity used, then the process design is in question. Since that is often not the case you should verify the part was dimensionally correct (Was it too thin?) before being peened. Next, double-check that your arc height measurements and intensity calculation are correct.
In the end, any corrective action would need approval from the part owner and/or design authority.
Please let us know your findings, or additional information. I would appreciate reading a follow-up.
EI SPT Director, Peening Preceptor & Product Engineer