RS250 pistons will fit, but they use a different sized small-end bearing/gudgeon (wrist) pin to the NSR.

The bearing you need is an HRC F3 part only, and extremely difficult to obtain. I know where there are 3, but they aren't for sale.

The problem with "alternative" caged small-end bearings is their speed rating. The NSR/RS bearings are rated to 14,000rpm, but "normal" bearings (from a factor, for example) are only rated to, if I remember correctly, 12,000rpm.

Andy.
NSR-WORLD.COM