Complex sentence: The 24th airframe of the O-43A contract was completed as the XO-46 prototype, with a revised wing and an engine switch, from the O-43's inline engine to a radial engine, the Pratt & Whitney R-1535-7.
Split into simpler sentences: The 24th airframe of the O-43A contract was completed as the XO-46 prototype. Main changes were to a wing construction, and inline engine was changed to a radial engine Pratt & Whitney R-1535-7.