Kawasaki's '07 KX450F is as modern as big-bore MX technology gets, thanks to a raft of upgrades to the frame, engine and suspension. Not content with that, Service Honda yanks the 450 motor, sets it aside for sale, cuts out part of the lower frame cradle, welds in a new cradle machined from billet and bolts in a KX500 two-stroke monster motor. The airbox is adapted and a new pipe constructed to fit the modern chassis. The conversion looks very clean.So why would you want a KX450F with its state-of-the-art four-stroke engine removed and a KX500 monster two-stroke engine added in its place? If you have to ask, you probably aren't a potential buyer. There are obvious signs that you are a KX500AF candidate: You're psycho about power, and you love to be different. You might be intrigued by a light, simple, easy-to-repair motorcycle. A small percentage of riders hold no four-stroke love in their hearts. They either fear the complexity and repair costs, or they simply thrive on the massive hit and brutal acceleration of a 500cc two-stroke. Even a muscular 450 like the KX-F feels tame by comparison. At low rpm, the 500 pulls smoothly and hooks up well, but as the rpm climb, the pull on your hands and shoulders is crazy, and controlling wheelspin in questionable traction is tough. Like the 450, the 500 is kickstart only, but the 500 requires jumping up and using your weight on the lever. Like any two-stroke, it starts easily once you get the hang of the kick. For riders groomed on thumpers, the 500 vibrates a lot with a heavy tingle through the grips, pegs and seat. As soon as you open the throttle a ways and the fear sets in, the vibration isn't that noticeable.The best place for 500cc two-strokes has always been the wide-open desert and fast motocross tracks that are sandy or deep and loamy. Having a high-tech chassis hasn't really changed that. This KX500AF feels solid, planted and yet more nimble than any steel-framed model. Our test bike's suspension action was not smooth or plush like a stock KX450F, and suspension tuning will be required for most riders. But to those who always loved their 500cc two-stroke, nothing else will matter once they whack open the throttle.