An SP is its bodywork.

Any model SP without the OE SP pannels is worth much less than one with it.

As it happens I have a full set of '89 Terra SP pannels but I'm waiting to find the bike to fit them too.

The last '18 I purchased was a '89 in almost perfect condition and untouched. I payed £800.

I would think with no fairings, and doubtful condition of motor and chassis I would be looking to pay a quater of that.

Re sale value. Well Less than a MC21 SP but cooler. '18 prices are alittle hit and miss these days.
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If I have to take the carbs off once more...