Cheat mode:
Type WIZ or WIZBOREWIZ at the second title screen, with the number of players option to enable cheat mode with invincibility. Press a number to start at the level last completed. 

Level skip:
Press 1 after losing with level 1 completed to restart the game with level 1 completed. Press 1 + 2 after losing with levels 1 and 2 completed to restart with both levels completed. 

Unlimited lives:
Load or reset the game, then execute the following BASIC command before running or restarting the program. 

POKE 27440,189 
Unlimited lives (alternate):
Load or reset the game, then execute the following BASIC command before running or restarting the program. 

POKE 27434,169 
Invincibility:
Load or reset the game, then execute the following BASIC command before running or restarting the program. 

POKE 32508,96