I really liked the game. It was fun and simple and I could see myself playing this on an actual nokia. In my opinion it was too easy. I died at 4000m when I stopped paying attention. I think some sfx could also help. At least a little chime every time you reach a km or something like that.
I think this game has a lot of potential if you decide to make a version without the nokia limitations. Also bonus points for using ebiten :)