Actually the reason why Japan cannot re-militarize is that it already has done so decades ago. Its military may be called "self defence forces" rather than "armed forces", but it really is a well-funded army of moderate size (6th highest military expenditures in the world).
The constitution MacArthur oversaw specifically forbids a military[1] and despite recent attempts to amend the constitution this has not happened. However, the limit on a military force was later interpreted as provisioning a self-defense force, ergo SDF. It was intentionally limited so as to prevent Japan from immediately rising as a military power but to not require longterm occupation of Japan by U.S. forces[2][3].
In practical terms there are only two restrictions on the Japanese military: No nuclear weapons and no aircraft carriers. Even the prohibition on aircraft carriers is a bit leaky.
Have a look at the Hyuga class "helicopter carrying destroyer": http://www.jeffhead.com/worldwideaircraftcarriers/16ddh.htm. On thing you notice is that unlike almost every other amphibious assault ship in the world the Hyuga class has an offset island, meaning you can land fixed-wing aircraft. Officially it carries 18 helicopters, but it has everything you need to carry an air wing (range, fuel, ammo, maintenance, and communications). So all they really need to to is fly off the helicopters and fly on some sort of STOL aircraft (like the F-35C) and presto! An aircraft carrier about on par with the British Invincible class.
Recently the Maritime SDF (you know, Navy) got the go-ahead for a "helicopter carrying cruiser", which would be the same thing only bigger, although still officially not an aircraft carrier.
The two you've listed and of course limits on national spending that involve the military which as a result significantly limit the growth of the SDF. Japan wants to avoid directly and blatantly invalidating their constitution; after all, it is supposed to be the fundamental set of laws that validate the state. That's why there's been such a push to amend the constitution.
