that piston has "caught" some time in the past- i.e. its seized. There is also evidence of "blueing" on the gudgeon pin indicating overheating.
From memory I think you had a 175 conversion done on that engine so you will need an oversize 175 conversion piston- not a TV175 piston as the gudgeon pin heights are different. Scooter Restorations do these pistons I think, and also good small end bearings.
Steve at Piston Broke knows his stuff- simply give him the cylinder and the new piston and he will bore to suit (3 thou clearance). It might be worth asking him to put 2 small holes on the exhaust side of the piston to aid cooling- hes done a few for me and it certainly helps.
Beware, Steve can talk for England.