This has happended many times before!

Definitely a seizure. At the very least you should take both cylinders off for inspection. Many posts on the forum with pictures of seized / damaged pistons / barrels, try doing a search.

And do consider, its not just a question of replacing parts, you need to identify why it seized. If not it will seize again.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30