The Head you'd have to get measured to see how much it had been skimmed to see how much it's worth........but generally looking £300-500 (£500 if totally unskimmed).

Crank, if it's unground then probs around £300. Rods, £80 for the set? Pistons you'd be lucky to sell mate, most people won't use second hand pistons if they have any sense (unless VERY low mileage) but stick em on E-bay you might get sommit for 'em, seen quite a few second hand cossie pistons on there in the past.

Cheers,
Ian