I really like your game very much. It's visually pleasing. I also like the music and background effects. It's really amazing what is still possible with the C64. It's fun to play, sometimes it is tricky. It really works best on real Commodore hardware; struggling more on the TheC64 (probably input lag of the system itself). Excellent work! I am recommending it to others.
I'd have two ideas for your game: Maybe have two or three different difficulty levels. And have checkpoints generate codes (letters and digits) which would enable the player to resume from that checkpoint even after power-cycling the computer. A similar feature is implemented for example by the game Grid Pix.
Maybe the code could also contain all of the upgrades that the player had by that point. I don't know whether that'd be easier to implement than a save to disk (and load again) feature.