Hi,
its possible but costly to change the piston.
its a aluminium cylinder with a steel tube in it. This tube has a nikasil plating.
you have to change the tube and replate with nikasil----- more than 350 Euro
Better to buy a new Kit cpl.
look for this - a more powerfull piece than parma :
http://www.sip-scootershop.com/Products/75039800/Rennzylinder+FABRIZI+145ccm.aspx
regards