Frankly, I think that if it were possible to put a biological brain inside a mechanical “body” that the person would go insane. None of the inputs and senses would make sense to the brain. The brain would be used to feeling a body, and the way the body provides information. At least some of the thinking goes on inside the body.
To have part of your brain replaced by something digital, that might not even be able to learn would drive anyone crazy. Even if you could translate digital signals into signals that might fool the nervous system into believing a body was still there, I don’t think it would work. Too many nerves. Too much processing power would be needed (even with a quantum computer, you might not have enough power).
Sorry. I know this question is about fantasy, not reality, but I couldn’t help but think of the technological and personal problems that would have to be solved for this to work.