Phil, a top-end rebuild kit for an MC28 (piston, rings, pins, bearing, gaskets) costs £85 in the UK.

Be careful with second hand pistons. I used to rebuild many 2 stroke engines (admittedly Stihl chainsaw & disc cutters and not NSRs), many times a piston looked fine on its skirts (no scores or marks), but the ring grooves wear which allow the rings to ramp up and down inside the grooves, which can then continue on the path to seizure.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30