Someone said (at least in some parts of CA) 1.28g is required now, and I think it was San Diego that was included. A call to your local building department should tell you if you can't find it online.
Zero clearance is only an issue with potential condensation with the tank tight against the wall - functionally, it would work, and, while the low-flush toilets often don't have a problem with condensation, it can still happen if the RH is high, the incoming water is cold, and the toilet is flushed before the tank water can warm up to room temp (course, if it's foggy, that may not be enough to stop condensation). Pretty much all newer toilets do not use all of the water in the tank, so mixing the refill water with the (hopefully) warmer tank water keeps it above the dew point unless it gets flushed repeatedly without time in between.
It might be tough doing something like change the flapper without pulling the whole toilet. Someone will be able to measure the thickness of the tank to give you an idea of what, if any, space is available, but I don't think it will be much, to get your arm in there.