Using Contemporary actors:
Blondie - Serious Answer: Michael Fassbender
Sounds Crazy, But Just THINK About It, Don’t Dismiss It Out Of Hand answer: Aaron Paul
Angel Eyes - Serious Answer: Jude Law
Sounds Crazy, But Just THINK About It, Don’t Dismiss It Out Of Hand answer: Jim Parsons
Tuco - Serious Answer: Demian Bichir
Sounds Crazy, But Just THINK About It, Don’t Dismiss It Out Of Hand answer: Mike Myers
All at pretty-much comparable ages to Messrs. Eastwood, Van Cleef and Wallach when they filmed TGtB&tU.
Using only other SW stars from back in the day:
Blondie - Gianni Garko
Angel Eyes - Even harder to replace than Clint, imo. Gut instinct says Gian Maria Volonté but he already had a juicy part in FaFDM and besides, I need to give it to Franco Nero as an apology for not casting him as Blondie. God, it’s difficult all this recasting, innit? It’s all bloody politics!;D
Tuco - Tomas Milian, who frankly I think could’ve played all three.