1.

Andy (from the pics and sites page) has modified his bike and here is the brief list:

CBR 600 front wheel, forks, tripleclamps and Telefix adjustable clip on bars.
Tapered headrace bearings.
Front brake calipers from CBR400, master cylinder from VFR400 with kevlar brake lines.
Discs from CBX750. 120/60 tire.
Rear wheel and disc from CB750, standard caliper with modified bracket, kevlar line.
140/70 tyre.
Q-tec rear suspension unit,
ATAC removed and Shepherd pipes fitted.
Harpowa reeds (stiffer) at the inlet and 118 main jets.
Racing spark plugs.
Polished frame and other bits.

2.

a Japanese NS400R enthusiast (from the link list) found the following parts in Japan:

Exhaust Pipes

First thing every rider wants is the exhaust pipes.
There are two manufactures in Japan that still produces pipes for NS400R.
One is Jha and this is still the most popular one.
With this exhaust pipes, engine can take easily up to 13000rpm!!
This pipe is design for high rev but the power in low rev decreases quite a bit.
So if you are a crazy super fast rider or if you race with NS400,
this pipe is recommended.
The price is US$1600, it's dead expensive but there is a good reason:
THIS PIPE COME WITH A BIGGER CARBURETOR!! It comes with 28mm carb, the original ns's carb is 26mm.
Considering that it comes with carbon silencer, it is not that expensive for what you get.

The other manufacture is Alpha racing.
They makes pipes, up step kits and fairing for NS400R.
Their pipe looks cool, like 3 pipes sticks out on the right side.
The price is around US$1000

Raised Steps

If you think the steps of your NS400R is too low, this thing can help you a lot.
These up step kits can change your shipting pattern.
You can have reversed shifting with those.
Beet is the most popular manufacture that produces those step kits.
The price is around US$300.
There are some other manufactures that produces them, but I can't remember the name...
I think it was Craftman that makes them. Oh, Alpha racing too.

Steering Dampers

Steering dampers, you can fit most manufacture's somehow.
But anyways, there are ones for NS.
NHK (or was it RC-engineering...?) makes one for your NS400R.
I think it was like 150 bucks...

Handle bar

Hurricane makes a nice handle bar for NS400R. The cramp and the bar are separable,
so you can replace the bent bar easily after a crash. I donno how much it was...

Plastics/Body Work

I only know that Alpha racing makes plastics for NS400R.
The thing is, they make fairing with FRP, so you can lighten up your bike a lot from it.
They have some race seat fairing and that,
you can remove that plastic seat you have on your NS400 and that makes a bit of difference.

Parts you can use for NS400R

Yeah, unfortunately, most NS400R parts disappeared, but you can use a lot of parts
that are designed for other Honda motorcycles.
Coz in Japan, there are a lot of 400cc bike and they often have
the same dimension and sizes as NS400.
Parts that are for NSR250 (this bike is like the second generation of NS)
are often good for NS400! The problem is that NSR250 was not exported.
VFR400(NC30), VFR750R(RC30) and CBR400R were exported to Europe but not to North America.

Air filters

Daytona and Kijima makes fiters.
You have to cut it by yourself to fit to your air-clear box.

Carbon Reed Valve

Daytona makes one but you have to cut it to fit to your NS400R.
They are 5 times stronger than the original reed valve and last long time.

Silencers

Daytona used to make nice slicencer but it is almost impossible to find it now.
Anyway, you can fit NSR250's silencer with a bit of modification on the mounts.
This thing is a lot lighter and boosts the engine a little.

Rear shock

Honda VRF400's (NC30) rear shock fits to NS without any modification.
It's harder shock but hell a lot nicer than the original one.

Brake - master cylinder

Front master cylinders from VFR400(NC30) and VFR750R(RC30) fit nicely
and you can get better modulation and stopping power.

Front End

You can swap the whole front without changing your stem.
CBR400R and VFR400R(NC21 and NC24)'s fork seems to fit without any mess.

Rear End

Swaping the swing arm and rear wheel is a bit of work.
You have to modify the pivot and the shock joint.
Most popular swing arms that are modified to fit to NS400R
are from NSR250 and VFR400(NC30)'s single swing arm.

3.

a German NS400R enthusiast found that following (moderate) modifications proved to work right:

Brakes:
mounting of:
Braking brake discs (front and back);
Braking brake pads (KC 10 front, SM15M back);
Lucas steel coated brake lines;

results:
        front: real good feeling for the brake, no fading,
                you need some hand force, brake pads need some temperature
        rear: soft and good


Engine:
precondition: crankshaft has to be ok (bearings, all oil seals);

moderate increase of compression (up 10%)
by milling 0.7 mm away from the cylinderheads (eliminates the funny edges) ==> CR around 7.3:1;
elimination of the restrictors (sleeves) from the 2 atac output channels;
mounting of a third rubber input hose to the airbox;
use of carbon reed valves (Carbonoracing);
use of Splitfire spark plugs;
use of nikon exhausts on cylinder 1 and 3 (for more power, better torque characteristic and better sound);
modify the original upper exhaust (2) by adding a 10 cm stinger and a nikon silencer;
use of stronger clutch springs;
use of Castrol RS racing oil;

results:
        higher torque, especially at low revs,
        about 10 hp power increase,
        mixture is slightly rich in mid range, a longer jet needle would be good,
        LESS fuel consumption !! and less weight (about 10 kg less),
        no smoke after 1/2 minute


Wheels:
mounting of:
CBR 600 F (PC19) front and rear wheels;
dimensions: front: 2,5 J x 17, rear: 3,5 J x 17;
tyres: Metzeler Lasertec (radial);
dimensions: front: 110/80 V 17 M/C TL (57V), rear: 140/80 V B 17 M/C TL (69V);

results:
        front: better handling than with original rims and the Bridgestone BT 45,
        MOT approval to be reached next week


handling bars:
mounting of:
Gilles Tooling type: GP light for 41 mm fork, using 2 mm spacer rings,
they are about 7 cm lower and have a wider angle than the original handle bars

results:
        front: better arm position, better seating position
        MOT approval to be reached next week

4.

Comparison of stock exhausts lengths with NIKON exhaust lengths


a. physical resonance lengths of stock exhausts:

exhaust on first and third cylinder with ATAC line: ca. 89,5 cm
exhaust on second (upper) cylinder: ca. 92,0 cm


b. physical resonance lengths of NIKON exhausts:

exhaust on first and third cylinder with ATAC line: ca. 97,5 cm
exhaust on second (upper) cylinder: ca. 69,5 cm

The different length of the second exhaust is the main cause for the difference
in the power characteristic between stock exhausts and NIKON exhausts.