I notice Service Honda are pushing 250 2 stroke conversions into late model chassis but this really only makes sense if you can detect the difference between last of the 2 stroke chassis and current 4 stroke ones. I think finally in the end the 4 stroke thing will generate just a bunch of dead rollers which you can't do much with that is really worthwhile.
Interesting. I can see there being a point in a few years time, but they way a 2006/2010 YZ250 handles compared to a 2010 YZF250 is far more about the differences in the engine than the frame.
Maybe if you're a dedicated Suzuki or Kawi nut, it might already be worthwhile?
I still suspect that swapping forks, brakes and rear shocks will get you 99% of the improvement of the later rolling chassis.
Funny how the car guys take old cars and jam newer motors into them, while the (dirt) bike guys pretty much do the opposite when we build era-irrelevant specials - we typically take the newest rolling chassis we can get our hands on, and jam an older (usually air cooled) motor in there.