If I had brain transplant and my body rejected it would you donate part of your brain you are not using and explain what it would do for me?

I'll just give you that part of my brain that has the explanation too. Then I won't have to explain it, you'll already know.

If I could, I would; but I think I'm using all of mine

And anyway, maybe my brain wouldn't help you.  I once made a comment to one of my friends< "You know how my mind works."

His reply---"NOBODY knows how your mind works."

I'll donate but can't make any promises as to what you'd get. You could find yourself in random places at random times breaking into song.

There is no part of the brain we do not use. The myth that we only use ten percent of our brains comes from Dale Carnegie's book How To Win friends And Influence People, where it was attributed to Lowell Thomas by William James in the introduction. When someone has a stroke, no doctor ever says, "It's all right. It's only in the part of the brain he doesn't use." It simply is not true.