A small number of KX450s have appeared spasmodically in pre '75 over the last 20 years and are reasonably competitive. There weren't many sold back in '74 so they are comparitively rare when compared to othe Japanese makes. The forks were made by Hatta and as Kawboy alludes to, were dismal at best. Kawasaki based their early advertising pitch around the multi adjustability of these forks, trying to convince us that they were the ducks guts when in reality they were bloody hopeless. The multiple adjustability didn't make diddly difference, no matter what settings were used. Even the awful steel bodied DT1 Yamaha forks were better. However with the availability of pd valves and cartridge emulators from YSS or Gold Valve, I'm sure you could get them working OK.
On the track the 450 has a surprisingly soft bottom end, possibly due to the light 250 based flywheels but it comes on bigtime on the transition into mid and last right through the range. They turn pretty well for a 70s era Jap bike but do suffer from a reasonably scary bit of headshake on long straights. Overall, they are way off the Euro open class bikes of the era but were pretty good when compared to the Japanese opposition. The ergos are great. Brakes and wheels are first rate but the shocks are shit but have probably been replaced by now anyway.
Another problem with the 450 was parts supply. Because the bikes were in such a limited release back in '74, the parts supply was also limited in relation to the small bike numbers and have long since dried up. I recall that at one stage during the early nineties one racer had a worldwide search for a piston and failed to find one. That was in the days before eBay and the emergence of the many www based vintage parts suppliers so the situation may haveimproved since then.
I know it's purely a taste thing and doesn't mean much overall but the thing that puts me off are the KX450s the cobby looks highlighted by the horrible pea soup green plastic tank colour. Thankfully forum regular Leith Codrington (LWC3077) showed us how to get the tank looking spic'n'span in VMX #33.
I think that it was Jimmy Weinert that rode works Kawasaki 450s (see below). The bike shown is very different to the production model. I may be wrong but I think that Brad Lackey rode a works rotary valved 500 based on the 350 Bighorn engine in '71 or '72.
