Some new cheats for you. All credit to Maxim for these.
Cheat Mode In the options screen, select Tune 09 and Sound 12. You can now select 'Cheat' in the Game Mode option. This will give you invincibility.
Level Select In the options screen, select Tune 10 and Sound 07. Move the cursor to Exit and press Left/Right to select your starting level. Note that the maximum level selectable is higher if the difficulty level is higher; and that Cheat Mode lets you access all the stages.