I too heard the same thing when I bought my 2000 model KDX 200. I couldn't find the stopper, so I asked the head mechanic at the edealership I bought the bike from. They had already removed the throttle stop and the restrictor in the silencer (muffler). He told me that it is a small white square peice of plastic in the thottle housing that can be drilled out.
By the way, have you made any changes to the stock jetting or exhaust pipe? I can't afford a new pipe, so I decided to rejet the carb to get a little more oomph out of the bike. I changed to a 155 main jet, a 45 pilot, and raised the clip to the second position from the top on the needle. I also turned the air screw out approx 1 1/2 turns. I also pulled the snorkel out of the airbox lid and drilled 5 (one cm in diameter) holes in the back of the lid. The new jets cost less than $25.
This made the bike run a lot better. It no longer felt like it was bogging down, down low and flattening out up top.
If you don't want to rejet, at least make sure your airscrew is not fully closed as mine was. Turning it out 1 1/2 turns made a big difference to the fact it was bogging down, down low.
I also noticed my clutch lever had no free play, this caused the bike to stall on numerous occasions under hard braking even though I had the clutch pulled into my fingers whilst braking. I would recommend giving a little more freeplay if want to avoid this scenario. (It's very frustrating when your racing and it drops you from 3rd to 10th position! :( )