Complex sentence: However, there is scepticism that the aircraft which was brought back from France was the Kalkdam original that had been modified: it was probably a copy of Farman's design, with very minor modifications made by Weston.
Split into simpler sentences: However, there is scepticism that this aircraft which was brought back from France was the Kalkdam original that had been modified. "(It) was probably built as a direct copy of Farman's design, with very minor modifications made by Weston".