250-500cc '07 WR144 to '13 WR250

After some consideration, (money) we are looking at staying with the Italogermanic Husky's for a couple more years. (yeah, I know, it's not a real word)
The question is what to expect to run into, switching from a 144 to a 250 two stroke, besides the obvious 5 speed tranny.
What mods are must do's for a 170lb young "A" rider?

We will eventually embrace the new Austrian Husky's but for a young college student, they are too expensive, assuming they are priced like a Berg.
Any advise will help, as we intend to pick up our next years race bike asap, before they are all gone.
PS. He really went fast on my 08 CR167 at the last two races and won his class at both.
Another 125 is also an option we are looking at.
Thanks in advance guys, you are always helpfull.

My vote is to stay with the better handling chassis- the x-lite. You need a 2014 cr144 with a 165 kit.

Five speed trans might be better for him in a race situation. I dont know much about the smaller motor but the 250/300 has a fantastic hit on the top. That being said I would add a steering stabilizer and a lectron carb for quick changes during condition variance

If he is winning on a 167 then my vote goes to a nice WB165 also, To my mind a far easier better handling bullet proof bike than the 250 but this is my view and Im totally biased just because no matter what I ride I still always prefer the tiddler 165

PS may change my opinion tomorrow as I out am testing some shercos.