Julian,
that is definately a detonation seizure. With your setup, most the NSR racers there use the HRC 0.6mm base gasket on the front cylinder to help prevent this. Looks like you were just a tad on the lean side with the jetting.

HARC PRO should be able to fix your cylinder. I believe they do replating. Rich had his RS125 cylinder repaired by them, and I believe it was alot less than the price of a new cylinder.

If you do get a new cylinder, just make sure it is off the same model bike as the other cylinder you have (ie, MC21R or MC28P, etc). The bore, A,B,C, will make no difference.
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Paul Herr
'88 FZR4/GSXR/YZF Frankenbike
MY BIKE PICS