I've had my friend's MT250 up to 70mph with no head shake at all. I'd say the front end geometry is more of a street-riding setup, with a lot of rake and slower turning response than an MX bike.
I'd check every single thing in the front end for looseness: steering stem, triple clamps, axle, bearings (wheel and stem), axle spacer, backing plate, etc. I would also check the swingarm pivot as well, for the same things. Problems out back can also cause head shake, especially if it's related to a chassis resonance.
For what it's worth, I always replace all the balls in the steering stem bearings, even if the old ones look OK. The new balls always roll smoothly on the bench, while the old ones often do that clunk-clunk-clunk crap. It would also be a good idea to put in new swingarm bushings, if you can get your pivot bolt out.
Ray