Go with the RB Carb and Head mod!!
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I have done almost all of bolt on modifications that are recommended for the KDX 220, and I love the way the bike has turned out. It is still friendly and comfortable, but everything has just been sharpened up. I too used forks off of a KLX300, I then had both the front and rear suspension tuned for my weight and riding level. The Gnarly Desert pipe on the 220 is great. I cannot say enough about the work that Ron Black does to the 220 carburetor. The throttle is super crisp now, and after I played with the air screw to find the " Second Sweet Spot" the bike has no hesitation. I would definitely go with the RB carb mod, and the head mod as well. So far it is all working great for me. BTW: If you choose to go the RB route...drop him an email and tell him about your bike setup, what kind of riding you do, and where you live. He has a very fast turn around, and does great work.
Good Luck!!