KLX 300? hmm...
The only problem with that would be where it would fit in the rules. (check out supertt.com for the rules). It's too big for lightweight and you'll be waaaaay at the back in middleweight (since you'll be racing against 400/426/450's). Although, this year may be a bit different since the YZ/WR250F's got moved out of LW and into MW. A buddy of mine had this bike and he sunk a ton of $$$ into it to try to get everything setup right for trail riding.
And yes, a flywheel weight will help the tractability of a 2 stroke dramattically (revs up slower, revs down slower) and will help it pull through the corner.
I bet you can get a used CR, lower it, put wheels on it and still be less money than a new WR/YZF.
While on the topic of wheels, you've got 2 choices to make it a real motard:
Option 1. (cheap route). Keep your 19" rear wheel. Find a dirt track tire dealer. Have your front wheel laced with a 19" rim (instead of 21"). 19" definately work better in the dirt than 17's, but 19" tires are not quite as good on the pavement (since they have a form of a knob).
Option 2: (better route). Take the stock hubs and get them laced up with 17" wheels. Benefits: better choice of tires, more contact patch, works best of pavement, works OK on dirt. My 17" wheels are 3.5" front and 4.25 rear. Since you have the hubs, brakes, etc already all you need is the rim hoops and spokes.
Have the CR dropped 2-3 inches. A good suspension guy can do this cheap (all they do is cut the springs and add spacers in the forks).