What have you tried or what do you think is a good way to go about it?

i think you would need an if statement so when the object is removed you would remove the actor and replace it with another one but i don't know how to make that condition or a if statement

Is the powerup to be used to change one type actor to a totally different type (like a whale into a bowl of petunias)? or, is to used to change some state (image, speed, or some ability) of the actor acquiring the powerup?

i would like to change the actor completely but if its too much work i would like to change its size or speed at the least

Alright. Either way is possible. Easiest may be to just go ahead a replace one actor with one of a different type (especially if the image changes also -- apart from scaling). The isTouching method is sufficient for the if condition. And, add the new actor into the world before removing the old one, so as not to lose reference to the world. Size and speed are, however, only states of an actor that can be changed without introducing a new type actor.