Wow, that one keeps one busy.
Nice little game, only quirk I see (at least on Xemu) is that input is sometimes choppy - some times joy left/right won't do anything, some times you jump by two fields. I have similar issues in my Snake game, I assume that comes from skipping N frames and then on a single one using the Joy command to check joy-state.
Not 100% sure though - ideas on how to improve are welcome ;)