I dont believe there are any missing pieces on the Death Star at all but if there is they are easily ordered from Lego for free. And the Princess Leia figure can also be ordered from Lego for free. The item "LEGO Star Wars Death Star (10188)" is in sale since Sunday, August 12, 2018.
This item is in the category "Toys & Games\Construction & Building Toys\LEGO Building Toys\LEGO Complete Sets & Packs". The seller is "chasingcars35" and is located in Leyland. This item can be shipped to United Kingdom.
Get on with the mission with this Star Wars brickset.