Hi, try as I might, I cannot find where I saw those Chinese pistons advertised, but the situation here has changed.
I pulled out of the shed an old 19J which a friend had given me as scrap years ago and, after I removed the head I discovered that the pistons were the Teflon coated ones with only barely detectable minimal cracks in them ! Looking further, the engine number was just 64 digits after the changeover to the 'improved' 19J - it just crept in by the skin of it's teeth !
That was the easy bit. Removing the sump and big end caps allowed me to remove the two inner pistons and rods but NOT the two end ones. Then I discovered why - the rods were bent ! Somebody must have terminated this engine by 'hydraulic(k)ing it in water ! I had (with great difficulty, as I didn't want to bother removing the crank), to angle grind and saw the big ends of the two rods through one of the bolt holes to allow the big end eyes to pass up through the bores and out.
Having done this I could examine the pistons very carefully and they look very serviceable indeed - certainly good enough for my limited and sympathetic use of the vehicle, so they're now earmarked for fitting to my engine - £160+ saved !