Why did Kevin make up a story about the bionic body?
There could be several reasons why Freak made up the story regarding his bionic body. After he dies, Max is clearly distraught, and even slightly hurt, that Freak lied to him. This is where the idea of a "justifiable" lie comes in.
Freak may have imagined this story in his head in order to protect himself and those who love him.
In one of my favorite quotes from Freak, he explains that you can remember anything, whether it really happened or not.
With this philosophy, we can understand why Freak may have made up this story in order to protect himself. He knows his time is limited, so in order to make the most out of the moments he has left, he makes up adventures to save damsals in distress and he makes up a story in order to alter his reality. With a bionic body, he is able to cope with his reality and fate.
We also understand why Freak may also create this story to protect his loved ones, but particularly Max. Freak may have been afraid that Max wouldn't want to be his friend if he knew that he would die soon. On the other hand, Freak may not have wanted to elicit the sympathy of others that Max frequently receives. Another alternative could be that Freak knew he could help Max overcome some of his personal obstacles before he leaves, therefore leaving his mark on the world.