Hint: 99 lives
In Zone 6, Act 1, go to the first Flybot Alarm and perform a Spin Dash, favoring the right half of the alarm, and hold it. If lined up correctly, the Flybots will keep appearing. Eventually you will start getting 10,000 points for each Flybot. Keep doing this until you get 99 lives.
Note: depending on how many lives you need to get to 99, you may surpass the time limit.