Okay, I'd use a variable here, I'll be using "CanUsePowerUp" which starts at 0
X is how long the power should be active
Events: On power up activated
CanUsePowerUp=0
Actions: Set CanUsePowerUp to 1
Set max speed to self.platform.maxspeed+50
Wait X Seconds
Set max speed to self.platform.maxspeed-50
Wait 20-X seconds
Set CanUsePowerUp to 0