Being the optimist that I am, I love to imagine the visit to the hospital (when Kevin is still alive) as going quite differently. Instead of Kevin having another seizure causing Max to leave, I like to imagine a friendly visit, a lull in the conversation, and Max's epiphany about the reality of Kevin's condition ending in a very simple prayer for a cure. Miraculously, there is no attack ending in Kevin's death, much to the surprise of the doctors. Kevin goes on living, depending on Max for help throughout their school and college years until the two end up becoming business partners in an engineering firm using Kevin's knowledge of robotics and Max's street smarts to create a wonderful life for themselves and their respective families. Together they continue to be "Freak the Mighty" to everyone they know and to those who simply know of their successful company.
On the more realistic front, I imagine the death of Kevin coming a little bit later in the story. Instead of Max returning to the hospital and to the news of Kevin's death, Max returns to the hospital to a gradually failing Kevin who is still able to communicate, talk, and laugh. Max's epiphany here would be the realization that his good friend Kevin is, in fact, going to die. The two of them could then discuss their adventures as "Freak the Mighty" and talk with the doctors about the lack of the real "bionic body" ... and end with the hope of others with a similar sickness benefiting from the idea created by the two of them. Max is inspired by this idea and pours his whole life into becoming a specialist in prosthetics and bionics (in honor of his friend), allowing Kevin to live eternally in Max's future work.