In a new video on YouTube, Barretto has shown off a handful of improvements including a complete rework of Mario's model - with some "improved proportions and better textures". The limbs of Mario have also been made "wider" so that they're easier to see at this low resolution.
In addition to these character model improvements, the animations have also been updated - with "more keyframes" - especially for jumping animations. Mario, in this GBA version, can also now perform more of his moveset from the N64 game including ground pound and wall sliding.
To top it off, there's been an update to the game's camera, transitions between stages when moving between areas, and stars have also been implemented. So, that's the latest on this developing SM64 project for the GBA. While there's no public version of this build available, it seems Barretto is open to sharing the game in some way or form when it's in a "better state".