Like others, I see some ambiguity here. Let me assume that the substrate includes not just the neurons, but the glial and other support cells and structures; and that there needs to be blood or equivalent to supply fuel, energy and other stuff. Then the question is whether this brain as a physical entity can function as the substrate, by itself, for high level mental functions.

I would give this 95%.

That is low for me, a year ago I would probably have said 98 or 99%. But I have been learning more about the nervous system these past few months. The brain's workings seem sufficiently mysterious and counter-intuitive that I wonder if maybe there is something fundamental we are missing. And I don't mean consciousness at all, I just mean the brain's extraordinary speed and robustness.