You can fit a bigger carb without an adaptor, I changed mine from a 26mm to a 30mm and it fit pretty well. I belive you have a PZ26 by default and a PZ30 should be a direct replacement. With a bigger carb you will lose some low end but gain some top end power. Getting the jetting right will be a little difficult however, it was fairly easy on my bike because I already had a 150cc big bore kit installed but with a 125 it might take a fair bit of trial and error to get it spot on.

http://www.ebay.co.uk/itm/Carb-NEW-C...item3a7bb66613