I have a set of 15mm pin A kit pistons which will fit the NSR stock crank. I haven't fitted them yet (had them 2 years!), but the cylinder will need shortening to get the timing and head volumes correct.

NX5 pistons may work, but you'll need to check ring end position to make sure it doesn't fall in the port.

VHM have flat top RS125/250 pistons that weigh nothing, use a 15mm pin and are horribly expensive! They also run a super low friction 0.7mm ring. They (VHM) also have dome top RS125/250 15mm pin pistons & 0.8 ring, but are out of stock. Again they weigh nothing. I'm trying to negotiate a decent price.