Try all the Suzuki dealers you can. One might have some NOS on the shelf collecting dust, and be only too happy to get rid! There are plenty aftermarket pistons being made too. Welcome to OSS by the way.
Well, you can’t do any harm by lapping with fine grinding paste. Try it, you’ve got nothing to lose. If you want to keep the Kat crank & rods, metal spraying/welding the taper might be an option … expensive, but an option nevertheless!