Machine the crown race down to the ISO diameter (26.4 mm) and use an ISO headset instead. The head tube diameter is close enough to be within tolerance. Or use the Park Tool CRC-1, details here.
Thanks. Bought a cheap one on ebay
No problem. What did you get? My tandem uses JIS (FFS), but I might get it machined down as I suggested to you.
@Hulsroy started
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Machine the crown race down to the ISO diameter (26.4 mm) and use an ISO headset instead. The head tube diameter is close enough to be within tolerance. Or use the Park Tool CRC-1, details here.