Suzuki RM125 pistons are a nice and cheap alternative for the stock Nsr250 pistons though some mods needs to be done. They are single ring pistons that are way lighter than stock but also not so trustworthy, so to speak. Perfect for racing/track use nonetheless.
Keep in mind that Rm125 pistons have about 0.6mm lower crown height than stock pistons. Also, check for any differences in piston dome height as there are a lot of different manufacturers for RM pistons out there.
Parts needed are :
1) Suzuki Rm125 '90-'99 pistons (obviously, I reckon).
2) Rm125 little end bearings, 15x19x19.5 (I think all years 'till now are the same but please first check).
3) You need to solve the difference between the stock and Rm piston crown height (if you do care about your compression ratio that is). There are three different options to follow:
a) Milling the cylinder base will get compression back to stock and will not alter port timings.
b) Milling the top of the cylinder is an option as well but not recommended due to high risk of pilling the nicasil plating.c) Machine the cylinder heads so that the combustion chamber is recessed into the cylinder as per the pics. This option will change port timings.
Have fun & keep the two strokes alive.