Any part that`s listed as `GY6` or `139QMB` will fit. The code on your kymco block is different but it`s a 1:1 fit with any GY6 block with only the attachment points (to the frame) being different.
A big bore kit will increase your CC`s and therefore increase torque and hp (top speed will be the same because you will hit your rev limiter).
Downsides can include:
High potential for crank damage/wear
Higher fuel consumption
It`s not legal
Can go wrong, can deliver some issues (most of the time it`s not the parts fault though.)