The '03 YZ450 is a lot like driving around town on a rainy day in a '69 Camaro with a 427 big block in it. Too much power too easily accessible too soon above idle. You sneeze, you do a burn out. The Rekluse will help with the stalling, and eliminate the worry about modulating the clutch properly, but that's it. The throttle still acts like a detonator.
The flywheel weight helps some with stalling, but it helps more with smoothing out the power flow at low RPM, so with or without the auto clutch, it's still beneficial.
Suspension is important alright, but it's expensive to do anything really effective to it, and the OP may not consider it worth while to spend that much on an '03. What can be done cheaply, though, is this. First, try a fork oil change, being sure to use a light grade suspension oil (~2-4wt) and keep the oil at near the minimum level. With the springs removed and both the damper rods and main tubes compressed, the oil level range is supposed to be between 135 and 85mm down from the top of the upper tube. Try it at 130. If you can run that oil height without excessive bottoming, the lower level will make the front end fell softer. Also, back out the compression clickers on the bottoms of the forks. Turn them all the way in just to where they stop, then come out about 15 clicks. If this feels too loose, bottoms too easy, etc., go back in two at a time. Loosen up the rebound (top of the fork) some, too. Lastly, be sure the spring rates are appropriate for your weight. Stock springs are intended for riders weighing around 170-190 in street clothes. Too far off from that, and the suspension will never work right until the springs are matched.