The pilot reported that the power checks carried out prior to the takeoff were satisfactory, but as the aircraft climbed through 350 ft the engine lost most of its power. As the winds were light, and there was no suitable location ahead of the aircraft in which to land, the pilot turned back towards the airfield. The aircraft was high as it crossed the threshold and touched down at a higher than normal landing speed. During the ground roll the nose landing gear collapsed and the aircraft tipped over onto its back. Both occupants were uninjured and vacated the aircraft through the normal exits.