yeah.. the bikes are generally real similar. The motors are not exactly the same. Port timing is different (PA50 I is milder) and there are likely to be other differences. So, measure and test carefully when swapping parts.
I don't own a PA50 I, but the Hobbit owner's manual (available through the Resources section .. on this page.. )
and searching these forums will be helpful since there are lots of threads about the PA50's.
The variators are different. There are a couple of modifications that can be made to them for the sake of swapping from one bike to the other and just to raise the gear ratio a little. (Search the forums and Resources section) The modifications do not involve changing any pulley.
The final gear ratios are different on the PA50 I and II but i'm not sure if this is due to the variator difference or due to different gearbox gears...
Aside from the engine/drive train, my guess is 95% of the parts are swapable straight across, like brake pads, frontend stuff, etc.