Yes, you could in principle create a dissimilar but isomorphic quantum system to simulate reality. My argument is that the real one will take less stuff to build by a very large factor, where the factor is large enough that "stuff" can be validly taken to mean any of matter, energy, or negentropy.
Phew, I'm relieved your argument isn't something like "a simulation would by assumption be 'grainier' than a natural universe, and so it would 'split' less often, and so have less 'subjective experience.'"
As to it being a gigantic pain in the ass to simulate an entire universe - sure, and it's unlikely that we're in a simulation. But ignoring units is typically only done when even the exponent is huge, since 10^10^10 meters is 10^(10^10 - 3) kilometers, which is still pretty much 10^10^10. On the other hand, it should only take some well-design...
I've written a prior post about how I think that the Everett branching factor of reality dominates that of any plausible simulation, whether the latter is run on a Von Neumann machine, on a quantum machine, or on some hybrid; and thus the probability and utility weight that should be assigned to simulations in general is negligible. I also argued that the fact that we live in an apparently quantum-branching world could be construed as weak anthropic evidence for this idea. My prior post was down-modded into oblivion for reasons that are not relevant here (style, etc.) If I were to replace this text you're reading with a version of that idea which was more fully-argued, but still stylistically-neutral (unlike my prior post), would people be interested?