Snarf, I just did a piston and ring replacement on my nephew's 2004? kx110, along with a general top to bottom servicing. If i'm understanding the part you're talking about (without looking it up), just take it out and soak it in carb cleaner for awhile then use a brass or weak steal brush to scrub it clean. Get all of the carbon off of it so it moves freely. Might need a pick on some of it. There's usually a cover of some sort them a small e-clip that attaches some linkage to it. If the bike's rings haven't been replaced in a while, it might be wise to do them now. If he is not racing, most of the gaskets are reuseable with a little spray-tack or Yamabond. Most gaskets are pretty inexpensive though. Flush the coolant then blow air thru it before you take it apart. 2-strokes are a piece of cake - really simple.
By the way, treat the carb just like one of ours, take it apart and clean each and every passage out. The float may not be adjustable, but be careful with it.