Mike,

Check it has OE bodywork, not the pattern body work from Hong Kong / China. MC21 OE top fairing should not have any vents each side of headlight.

For an SP, it should have Magtek magnesium rims, it says Magtek on one of the wheel spokes.

Check for signs of crash damage, repairs to bodywork, marks on frame / swingarm where bars / footpegs have been pushed into it.

A 1991 SP should be in Pentax colours, the respray may be something completely different.

Check if its on oil injector or running premix. If premix, the top oil reservoir on RHS of frame will be removed.

Check it starts, from cold & warm.

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