As Fontyyy said, get the barrels off and inspect the pistons. There is obviously something wrong for it to lock up, and it will happen again unless the fault is rectified.

Some piston diagnosis info here http://www.smellofdeath.com/lloydy/piston_diag_guide.htm

Good luck.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30