What's failing on the CRF150 motor?
400cc two strokes are the best,love my 380 Maico. 
The 550 KTM is a far better motor then the 440 version, the 440 actually lacks some bottom end while the 550 is both very smooth and very fast.I'd use the 550 motor for AF conversions over the Honda anyday (the right hand final drive would complicate things).The 550 doesn't have a counter balancer but vibration isn't a problem.
They are about as bullet proof as a xr200 no problems, that i have seen ? i put a crank in a 2007 2 yrs ago that had been modded , with 3 race yrs on a engine with only piston changes thats pretty good i would say . But that 250 engine in that chassis would be good Dune material !!