With the cost of new pistons &  finding a machine shop to hone or bore your cyclinders why not upgrade & add more horsepower? Your R75 will go from 48 HP to 65HP. 
 
Complete psitons with rings
New cylinders with stainless steel pushrod tubes
complete top end gasket set
Nikasil coated cylinders
Lighter (150g), spontaneous quiet running pistons
Designed to require no further modifications to the head.
 
The R65 kit takes you from 650cc to 860cc