Complex sentence: The design was derived from the Bristol Britannia transport, having the same wings, tail surfaces and landing gear except for being "Americanised"-meaning that it used the same general design, but changing the British materials and standard parts for North American ones.
Split into simpler sentences: The design was derived from the Bristol Britannia transport, having the same wings, tail surfaces and landing gear except for being "americanised". Americanised means using the same general design, but changing the British materials & standard parts for North American ones.