Yes cataract surgery is possible, though the chances of complications are higher. Most of the patients can be managed by putting them in the Trendelenburg position. Otherwise different modalities like face to face surgery can be attempted wherein the patient is sitting upright. More information is available at http://eyewiki.aao.org/Positioning_for_surgical_patients_who_cannot_lie_flat

Yes. We can perform surgery. Doctor should modified the bed

Cataract surgeries have been performed in upright positions also though the technique needs a slightly better skillset. And also, the ease and safety would depend on the technique used.