This is mostly based on looks. Sorry I don't get how to paste images on this site so you'll have to use a search engine and images for my pics:
Katara: Kristen Kruek (of smallville fame)
Aang: Boo Boo Stewart (closest pick so far for me, also he does martial arts)
Zuko: Taylor Lautner
Master Pakku: David Carradine
Monk Gyatso: Gordon Liu
The singing hippie guy "secret tunnel": Peter Dante (Grandma's Boy "Dante" and other Adam Sandler Movies)
The Boulder: The Rock
That's all I can think of for now!