I think you have the old 2-stroke version. There is also a newer version of the FS 120 with stratified scavening engine (2-Mix from 2016) in some markets availible. Both of these FS 120`s have a 35mm piston but different piston rings. Have a look what your serial number is and let me know. Both versions have the same displacement.
For the old 2 stroke version you need piston rings 4119 034 3001 (35mm*1,5mm).