BarnBurner is mostly correct. the CB/CL 175 and 200 share the same bore, but the 200 has a slightly longer stroke. Other differences include the clutch, cam shaft, and carbs - but most of the engine is the same. The 175 actually has more HP because of the bigger carbs and more agressive cam. You can't put the cam from the 175 on the 200 though, because the timing would be completely wrong -- different number of teeth on the cam chain sprocket. (trivial info for a CM200 owner, sorry)
the CM200, on the other hand, completely different.