Complex sentence: An enormous amount of work was done over 7 years of scrupulous effort to recreate the historical appearance of the museum: this was the first full-fledged restoration made in the museum in its entire 200-year history.
Split into simpler sentences: An enormous amount of work was done over seven years to recreate the historical appearance of the museum. This was the first full-fledged restoration made in the museum in its entire 200-year history.