Tip: When determining a time limit, remember that players won't be as familiar with the level as you are. If you normally complete the level with around 100s left on the timer, others might run out of time on their first try.
This patch hijacks Magikoopa's "turn block into sprite" routine so that you can specify your own sprite numbers for it to generate. (There is a 256-byte table in the patch that holds all possible sprite numbers.) You can also use custom sprites and stunned sprites.