I had a GM actually do this for us. Attributes and Standings worked the same way as in regular MAG, but there was also a "Bending" stat that covered most of your elemental bending. Strong Powers gave you 6 Bending in the element of your choice(No being the Avatar), Medium Powers gave you 4 bending, and Weak Powers gave you either 2 Bending or no bending and the two free Traits (your choice).
Stunts were a lot more important in this game, as they allowed you bending tricks that you didn't otherwise have, or else greater bending abilities. For example, two stunts my Strong Firebender picked up were "The Flame Shall Not Hurt Me" (basically a total fire immunity, which allowed me to set myself on fire at will and not be hurt) and Greater Flames (gave me +1 base damage on Firebending attacks, also a pre-req for Azure Fire, which would have given me another +1 but I didn't get around to taking it). Or for another example, the Waterbender had the Healing stunt to allow him to use waterbending's healing specialty.
You could also use Stunts for other abilities. For example, another player (with no starting Powers) took the Chi-blocking Stunt to use that art (in this system, when you chi-block someone you decrease their bending (temporarily) instead of their Health).
Finally, the game included an option to "Snap" by spending a bunch of Advancements, and about 2/3 of the way through the chi-blocker I mentioned above unlocked Energybending that way.