Thanks Mate,
There is a standard piston, rings and bore on Ebay at the moment and it's quite close to me so that's an option. I will check the ring gap anyway but the rings didn't have much tension when I lifted them off. It did run well though, the tapered sleeve on the bottom of the crank is very beaten up. Is it the same as the later ones on the Specials ? Now the good news

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And some axles to make
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I will know what's under the paint when I strip it off, underneath looks all good :-)