As a last resort Martin, perhaps the piston could be "cherry bombed" to see if it will "grow".

I have copied this extract from an article concerning building a replica taplin twin prototype published in model engine news. The author is not named.

"... George Aldrich knows a LOT about speed engines and tells me he "grows" cast iron pistons a couple of thou by heating to red heat and oil quenching. On the theory that this pair was stuffed anyway, I gave it a try on the worst one (the front). "Before" measurements gave a diameter of 0.5044". The piston, held on a piece of soft iron wire was brought at a bright orange in a propane tourch and quenched in chain saw oil. The "After" dimension measured 0.5060" with no appreciable out of roundness. A little honing soon removed the scale and produced a good fit in the front cylinder. The piston is still not as polished as I'd like indicating just how under size I'd turned it (and how blunt the tool had been). Reassembled, the front pot would now "pop" on a prime. With this encouragement, the back piston got the same treatment with similar results: growth of 0.0016" and no distortion. Simply amazing. With both pots a-poppin, I just had to try for a run."