To get the skydome back, go into the Content Browser and select “View Options” (#1), then from the menu enable “Show Engine Content”. With enabled select the “Engine” folder (#2) then in the search box at the top type in “bp_sky” (without the quotes) (#3).
All you need to do then is select the asset and drag it into the viewport, you may need to adjust some of the values using the details panel to get it back to looking the same, but is a simple way of doing it. 
