About the only thing I'd do is add bark busters and a skid plate.
I guess it depends on how serious you want to be as you could add a flywheel weight, different tires, and suspension changes.
For a 250 2 stroke
bark busters
cut bars down
slightly lower gearing
3 gallon plus tank is defiantly a need.
spark arrestor
kicks stands are awesome!
some kind of pipe guard
suspension can be dealt with by adjusting, but it will always be harsher
many prefer the 18" wheel, but My buddies never have a problem with their 19"s.... (no trials tire though...)
fly wheel weights are nice, but not a necessity, especially if you are a faster rider..
headlight kits are also nice, but not needed by most people
This is for a more serious woods rider in my area, dez and quad trail riders may have different needs