Complex sentence: Each of the survivors of the 145 Electras built to that time was modified at Lockheed's expense at the factory, the modifications taking 20 days for each aircraft; and the changes were incorporated in subsequent aircraft as they were built.
Split into simpler sentences: Each of the survivors of the 145 Electras built to that time was modified at Lockheed's expense at the factory, the modifications taking 20 days for each aircraft. The changes were incorporated in subsequent aircraft as they were built.