theres a lot of little differences..
The PA50I has milder cylinder port timing.. Stock compression ratio is actually _higher_ on the PA50I than on the PA50II. It's rear wheel gear box is also geared lower.
And then there's the variator.. Personally, i'd avoid the popularly suggested method of machining the PA50I variator to get a higher gear. Any mistakes can't be corrected.
Essentially all of the important things regarding top speed have to be modified to transform a PA50I to a PA50II and isn't worth the effort, imo... but it would be nice to have a PA50I as a spare-parts bike.