I feel like the players should play themselves. They all seem to have their own charisma which none other could replicate. El, Rip, Sir Walter Ray, and especially the younger players.
Coaches are another story I suppose but even they are hard to duplicate.
Ed Norton Jr as young JC. Just popped into my head. Gene Hackman could play JC but Gene's a little too old now. Dude was born in 1930!!!
The only doppelganger I see is Barry Pepper and Joey Californication but Joey is still much younger so go with Joey Cajones.
For levity, Jonah Hill as George Blaney. Or Stephen Root. I didn't even know Stephen's name until I googled him tonight. He was in Justified and News Radio.
This includes a clip from News Radio, classic Stephen