Really cool! Just completed both levels on my GBC. Was very faithful to the original, and ran perfectly. I could image the original game the whole time wherever I was. Hope you can add some more music, as cool as the Halo theme is, it got a bit repetitive. Also some more weapons, maybe plasma pistol (charging) or needler (homing) as timed power ups dropped from kills could work on Gameboy mechanics? And nades of course! But great job!
Also, I am sad there wasn't the Cortana dialouge for this line in the first level