Hmm... Good question. Lessee...
Apollo: Ben McKenzie (Gotham)
Starbuck: Timothy Olyphant (Deadwood/Justified)
Boomer: Anthony Mackie (Captain America)
Adama: I had a really hard time with this one. Lorne Greene is such a difficult act to follow, combining caring father figure with religious leader with military leader. Olmos fell short as a father figure (intentionally) and left the religious aspect out of it entirely. He was really first and foremost a military leader only. Very few men currently of Greene's age come to mind for this role. Perhaps maybe Rene Auberjonois or John Larroquette (both of Boston Legal/Star Trek).
Tigh: Harry Lennix (The Blacklist)
Athena: Jaimie Alexander (Thor/Agents of SHIELD)
Cain: Jeff Bridges (he looks more and more like his dad every day) - or if not him, then Ron Perlman (Sons of Anarchy)
Baltar: Ian McShane (Deadwood - I can just hear him screaming, "Lucifer, you c*cksucker!")
Serena: Jennifer Morrison (House, Once Upon a Time)
Sheba: Megan Boone (The Blacklist)
Boxey: Written out of the story entirely
President Adar: Shelley Berman (Boston Legal - nobody plays a vapid old fuddy-duddy like this guy)