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Not necessarily . OEM work fine, but you should update the brake lines with stainless steel ones or better.

i Think it might be prudent to point out here for the viewers who are not necessarily that brake savvy why this thread is rolling along
i dont believe it is good to simply tell people they should do something without giving them a reason why
simply with regards to brakes this issue only becomes a issue to a individual either has a fault occurs or through riding ability you out grow the braking capacity of the machine, or as in a lot of our cases we decide to take newer technology and retrofit to old machinery that did not have the advantage of present mechanical understanding.
im sure their will be some still riding their much loved Z,s wondering why people are trying to change their brake set ups as they feel theirs works great!
this i believe ive covered above, for those who for what ever reason are interested in their brakes but dont know where to start,i would suggest you think about why you want to improve your brakes and then explore all avenues before you start
the easiest and cheapest start point is sintered metal race pads, these if available work significantly better than the usual factory pads, they will however wear out quicker
second is Braided Brake lines, the reason for using these is simple!, braking works on friction between your discs and the pads, this creates heat which transfers to the calipers and then the fluid in them, once the brake fluid starts getting hot it will soften the standard brake lines, so instead of the fluid exacting pressure on the pistons only, it starts to expand your brake lines as well creating a spongy feeling under hard consistent braking, a situation some riders may never suffer on the big open highways of America.
so using steel braided lines prevents the lines pumping up and the vague spongy feeling it does not improve the braking ability of the standard set up it merely makes it consistent. (though you may consider that in itself a improvement.)
but if you are thinking as some of retro fitting entirely different calipers master cylinders etc, then i would advise you do that first then get braided lines made!
when fitting completely different calipers you may need either longer or shorter lines to them, they may even require a change from 90 degree banjo,s to 45, or other angles, hence my remark decide first what you want to achieve, save the agony of wasting money.
you can retro fit virtually any modern caliper to your bike ,Nissan/Brembo/Tokico, you will need to check the bore/stroke ratio of the piston in your master cylinder with the manufactures requirements for the calipers to get the same brake set up they where designed for, in some instances you may well be able to just keep your original master cylinder, this becomes the riders preference in some respect! as what you may accept as braking performance and feed back may be more or less than than next guy.
ideally if you find some one who produces caliper mounting brackets for a particular brand it will be the easiest way to go WeeBike have mounting brackets for retro fitting Brembo Calipers and i did find some for retro fitting Tokico somewhere on the net.their is also the guy in this site.
using a good fully synthetic brake fluid will also help with longevity of the internal parts of your braking system, mineral based fluids are prone to adsorbing moisture which in turn brakes the fluid down and causes rust on any internal parts, synthetic fluid does not attract this problem, use dot 4 or above.
the last but not least is the discs themselves, make sure yours are withing manufacturers measurements and not warped, i think EBC brakes make after market disc,s with carriers.
you may also consider if you are chasing out right braking performance, the better they get the more you may need to address your suspension
good luck all
Dion

You are exactly right Dion. I've heard stories of people don't liking the results after simply retrofitted a master cylinder from another more modern bike (that wasn't on KZ).
An example somewhat related. A GL 1100 from 83 has an interesting brake system - the hand lever activates the from and the back simultaneously. Some people split that connection wanting the brakes work like a 'real' bike. In my case - I love it just the way it is (the bike stops in the rain 'like a car').
So, it's technical and personal preference. Many people just don't ride their bikes unless on a dry and even better sunny warm weather. The bikes never see rain and are ridden like 'if stolen'. I can see them wanting robust front brakes. In my case, I have ridden even with snow flakes falling, and in rain many many times including fast moving highways in heavy downpour when you can't see much in front of you riding in traffic at 120 km/hr. I do not like hard front brake, and they would be dangerous and disastrous in some conditions and situations I have been in. So, again, it's the pure technical mechanically and technical otherwise mix. Sometimes 'softer' brakes are better, in other times you just have to develop a strong hand grip.
One thing I suggest as a good brakes upgrade are those braided lines, explained above well by Dion. They improve control and help when you need sharper braking power.