very good but..
1. Make player move a bit faster.
2. Either give more lives or give the player a health bar.
3. Make bosses at end of stage shoot just a little less. It gets to be quite difficult to dodge all that.
4. What happened to the other weapons? I'm not sure if the game you are basing this on had more weapons but this could use some. If not different types, then at least make the weapon go higher than the three shoots and two missiles.
5. Rating system is kind of flawed. I got a B on level 1 and 2 while starting level 3 with no bombs and no lives left. You would think that a person receiving a B would be doing better than I was and would have a clear shoot at winning or at least getting far in the game.
6. Have player start a new level with 4 bombs. I think it is too difficult to have to use all your bombs on the boss, and then start the new level not having that supply replenished.
7. Some normal enemies through out the stage have just a little bit more health than they should. With the highest weapon I still spend more time on a single enemy than I should. Lowering their health some or allowing for better weapons would solve this.
Other than these the game was great. Very nice retro game to waste some time on.