Continue game play:
Hold A + B immediately after the game over screen appears. In a two player game, press A, B(2), A.
Level and difficulty selection:
Select a one or two player game, then hold A or B and press Start to begin game play. Alternatively, hold A + B and power on the NES. Continue to hold those buttons and press Start(2).
Options:
Press Select + Start + B at the player number selection screen. Press Up or Down to select an option, press A to change the current option, and press Start to begin game play.