With 2008 KX450F forks and a 2008 KX450F front axle (with stock spacers and nut) you can use your stock wheel. My wheel is from a 2002 KX500. The forks bolt right up in the triples, and the axle fits the 2002 wheel fine. Looking at the front of the bike you will take a stock 2008 spacer and put it on the axle on the right side. It will put your 2002 wheel in perfect alignment. When you install the stock 2008 spacer on the left side the axle will not come out enough on the right side to get very many threads on it with the stock axle nut. You will end up machining the stock 2008 left side spacer (both 2008 spacers are the same length) and taking some off to get the axle to move to the right to pick up some threads. Make the nut flush with the axle bolt on the right side, you will be good.
You can also use your stock front brake caliper, but will need to take a little bit off where the caliper mounting points are. Do not remove material from the fork side mounting points. You can actually do this with a fine file and go slow. Then your stock 2002 front rotor will sit right in the middle in your stock 2002 caliper. If you do not remove some material the rotor is off too far to one side. You will see what I mean when you bolt it all up. I was actually going to have Sandblaster or a machine shop do my caliper, but it worked so good I just left it. Have at least 20 engine hours on this setup, and it works great.
Normal disclaimers apply, and the years are different, Sandblaster may know if 2012 is the same as 2008 for triple, etc? 2012 probably has a better front brake, though I have never run out of front brake before.
Good luck, I will remember to check back if there are more questions. PM me if you need help.
