trouble shooting gearing issues on the 650 can be a bit puzzling at times.
I would start by looking at someof the docs in the knowledge base
, lots of good info there.
The pathway of the drivetrain is: Engine > CVT > Clutch > Transmission > Final Drive > Rear Wheel
no two bikes are alike and parts wear differently on all of them.
if the scoot revs high and it gradually engages and you can attain normal operating and gearing, it could be a clutch problem, or
the PPS could not be working properly.
hopefully the wrong kind of oil was not used, with friction modifiers, in the past which will most certainly affect the clutch plates
and require replacing them.
There can also be something wrong with the CVT, the 05+ models have a better primary pulley adapter design but there have been
a few failures there too.
I would start by performing an all around oil change, engine/transmission with a new filter and the final drive.
if the problem doesn't go away after the oil changes, you may have some sort of mechanical issue which will require
a bit of investigating, which means taking off a few pieces of tupperware and pulling some parts off for inspection.
you can check my burgman center if you are a DIYer for all sorts of DIYs to deal with the PPS / Clutch / CVT etc...