fly wheel weight

Putting a 13 oz weight on will help but probably not what your looking for. The KTM's and GasGas's have a much smoother power, not necessarily less just more linear. The flywheel weights are not to hard to do, a shop would likely charge you an hour labor. Worst part is getting the stock one broke loose.
I run an 11 oz weight on my 05 and it helps but it's not a putt-putt bike at all.

I have a 97 CR250R 2-stroke, and I put the max weight- 14oz on it. The FWW didn't make it a putt-putt, but smothed the power delivery out on the bottom end enough to make the bike rideable through the woods in 1st gear to pick through the rocks and trees.

It also gave the motor enough weight on the flywheel to make starting a little eaiser (in my opinion). I didn't see a change in the overall power of the bike, and most folks who borrow it comment how "rideable" it is for a 250 2-stroke.

I've never added a flywheel weight (KDX has enormous flywheel) but 14oz is a small percentage of the weight of that big diameter crank so don't expect a big change, also a powerful 250 2 stroke isn't going to see a bit of rotating weight as much of an impedament to rapid acceleration.

It might be worth looking at having the cylinder ported. A two-stroke can be modified for performance by adding epoxy to the intake tract in order to improve port velocity, and improve low-end throttle response. A port shop can also lower the intake port position in order to move the torque curve towards the low-end and mid-range.

A different exhaust pipe will also greatly change the characteristics of the engine: Look at the FMF and Pro-Circuit series of pipes. They both make a couple different pipes for optimizing where and when the power is made.

Between the flywheel weight, a pipe, and cylinder porting, you can probably make that bike into a tractor if you really want, and it'll cost you about maybe $500 or $600 for all three, assuming you turn the socket wrenches yourself.

Fine tuning the jetting on a YZ250 makes a big difference in smoothing out the power. They come jetted very rich and do not have nearly the bottom end they are capable of, stock.

Do a search for "jetting," and you can read up on what you need to do. It will take some of your time, but not much money. Great performance gains to be had, especially in low-end and midrange.

Once you get the jetting close, you might want to have Eric Gorr modify your cyl. head. This will allow you to lean it out some more, while avoiding detonation.

The 13 oz. FWW will help, as well. Good luck!