>>91
That would depend on the particulars of the laws of physics. Someone existing within some particular mathematical structure cannot actually change it, as they are themselves part of the structure. Of course, if whatever laws of physics allow for such an act, sure, it just doesn't seem like our own have any such possibilities (although technically, there's the quantum foam).
A belief in computability (thus natural numbers) in the ontology just means that there's a lot of possibilities as far as laws of physics go, as well as an inevitable first-person indeterminacy regarding in which particular state (or universe, if you want to use that term) you happen to be. It even tells you how you can change your "viewpoint" to one which you might like more (read that novel I mentioned for an example). However, getting one particular mathematical structure to be something else is of course impossible, for example: given the the usual definitions of Peano Arithmetic within the standard interpretation of arithmetic, it's impossible for 1+1=3 to be true (if PA is consistent). For some other system with very different definitions and semantics of 1,3,+,= or what one means by truth, that sentence may very well be true.
Belief in some countable infinity doesn't mean that suddenly the world breaks or that you can do impossible things.