Burt Reynolds as Han Solo
Jodie Foster as Princess Leia
Donald Pleasance as Obi-Wan
Marlon Brando as Tarkin (All his scenes would have had to be filmed in five days and he'd have received $5 million for the role.)
Orson Welles as Darth Vader (no suit, and he'd refuse to be shot with Brando, so all the scenes with Tarkin and Vader would have to be assembled cleverly in the editing room)
Roddy McDowell as Cornelius Chewbacca
Wilt Chamberlain as C-3PO and Billy Barty as R2-D2 (both would be recognizable behind their semi-robotic makeup)
ABBA as the Cantina Band
Filmed on the 20th Century Fox backlot with On Location shooting in the Joshua Tree National Forest for the Tatooine desert
Sets would look like "Space: 1999" and the FX would look very Planet of the Apes
Lightsabers would have been cut in favor of a deadly card game between Welles and Pleasance. But the fist fight in the cantina with Reynolds, Garrett and a cast of wacky aliens would be legendary.
Marvin Hamlisch would score the movie with cheesy Bee-Gees-style music (meaning he wouldn't have done The Spy Who Loved Me. John Barry would have done that score and it would have been good).
The most enduring thing would be the hot sex scene between Luke and Leia.