Could be a lot of things.
The piston from a water cooled engine might work, as long as it only gets the heat and forces that it was designed to get, and it never saw any detonation or lean conditions or wrong timing.
However the other possibility is that with a 90mm bore, that is FAR too much bore size, and left you with a thin cylinder wall which probably severely distorted when it got hot, and no piston can run in a distorted bore.