rider  fork      shock
-----  ----      -----
145lb  .40kg/mm  4.4kg/mm
160    .41       4.5
175    .42       4.6
190    .43       4.7 

Shock linkage "dog bones": These come with the correct length for a steeper-than-normal front fork angle which gives better sharp turning ability for use in the woods but less high speed stability. That characteristic is enhanced by a shorter-than-normal wheelbase (which also increases sharp turning ability). So unless those are changed for longer ones, the handling will be "busy" on the MX track with a tendency to loose straight line stability which may result in swapping ends and/or doing a tank slapper. For the latter years of the KDX, longer dog bones are available from aftermarket suppliers, but not for the older models. I have an '89 model and I just made some out of steel bars. After playing around with different lengths I settled on 3mm longer. At 4 or 5mm the rear wheel will hit the rear fender on landings due to the longer bones letting the rear wheel sit closer to the fender. This is a very important mod since how a bike handles is of utmost importance on a MX track. You can buy links from Kouba for a 95 and up KDX200 or 220 for $75 (order the KDX1).
Exhaust pipe
The first thing to do is replace the exhaust pipe unless you are really tight on dough. If so, then just take the power-robbing insulation out of the body of the pipe (not the header. see http://justkdx.dirtrider.net and click "tech tips", then "pipe mod". This mod is for years '89-'94). Click here for a good video on the subject. FMF has pipes for better low end and better top end (35 (torque) or 30 (rev) gnarly series pipes) but many have complained about quality control problems and mounting points not being correctly located. I chose to use the Pro Circuit pipe which enhances the high-mid and top end power. I have been happy with it. Recently I have gotten into expansion chamber design and here is a page explaining how I would custom make a pipe for it: click here
Reed valves
Next things to do is replace the stock reed valves with fiberglass Boyesen Super Stock Fiber Reeds for $30 or Boyesen Power Reeds for $30 or carbon fiber Boyesen Pro Series reeds for $45. Carbon fiber reeds are lighter and open/close faster which allows more air/fuel in the engine at high RPM.
Increasing air intake and preventing water intake
Taking off the stock "snorkel" allows more air intake but also will allow muddy water in (that gets splashed up there) which can cause your throttle to stick open. People do it to allow more air in which aids power output. But the snorkel is there for a reason. Too bad they didn't design it with a more voluminous intake. Once the muddy water cascades into the air box the air filter will pass it in because it only catches "dry" dirt particles with the air filter oil. The muddy water gets up there when it gets tossed "up" there while you are splashing through muddy water. My solution is to toss the restrictive snorkel and cut out a bigger rectangular hole. Then use something (I used cut out pieces from a quart sized mineral water bottle) that you can duct tape in place to build a 1/2" tall "wall" around the big hole to prevent creeping water from coming into the air box. Also you'll need to duct tape the front edge of the cover to the air box below. Also run a piece of duct tape from the left frame tube to the right one (taping to their undersides) in front of the air box which makes a splash guard there (behind where the frame makes a "Y"). Also tape the right upper side of the air box to the frame, effectively making a splash guard for that vulnerable side. These two mating surfaces let splashed water in. Also put a plastic splash guard under the area that's open between the tank and the frame. Also if you ride through deep streams you should silicone seal the air boot to air box juncture. If you have already drilled holes in the air box lid for greater air flow then this fix won't work for you and you'll need to buy another lid (they are cheap enough) and do it right. After all these fixes my air box still would ingest a little muddy water (but much less than before) from going through muddy puddles. So the final refinement was to put a piece of cardboard (sealed from water by putting packaging tape all around it) from the forward air box opening (secured in place with duct tape) to the near-bottom of the air box which basically provides a "chute" for the water to go down without getting onto the filter. Also I made the sides of the cardboard (about 3/8") bend up by 90 degrees (and held in place with tape) so that water won't get sucked over the sides of the cardboard and onto the filter. That way the muddy water just goes all the way down to the bottom of the air box. I finally put my noggin to work on this problem after I lost the last MX race due to the throttle sticking open with dirty water getting in. It was disappointing because I was going to win and it was going to be my only win of the year. I got 2nd place the first moto and then pulled off the track with this problem while in 1st place in the second moto. I was all prepared except for this one weak spot - water intake prevention. Also I've put silicone seal on the top of my carburetor to keep water getting in there where the cable sheath rests against the carb top. There's a rubber boot there but I think water could still get in.
Ignition Timing
My '89 model has ignition timing set at 21 degrees BTDC at 6000rpm whereas the years after had it set at 19 (with a resultant 1hp gain on top). After hearing that you can change it (if you have a flywheel puller and holder) to enhance performance I experimented by advancing the stator one notch CW and then CCW. CCW gave dull low end performance with just a little more power on top (but nothing significant), and adjusting it CW (advancing the spark) really gave it more umph in the low and mid range without detracting more than a little bit from the top. With it advanced I really like the increased power although it runs a little hotter. So with this mod you need to have a clean cooling system (use radiator flush) and use 50/50 antifreeze/distilled water to prevent coolant loss due to boiling.
Later I started tinkering with changing the timing curve by changing the CDI. (I'm an electronics technician.) I came up with two internal changes that made more power all through the rev range and even extended the rev range. Check it out here.