IIRC the side panels are about £70 each and possibly £100 for the rear subframe plastics.
Throw the standard front mudguard in the skip-it's useless and will just allow the radiator to fill up with mud, and either adapt a £20 ktm pattern item to fit, or get one of stefans very nice custom fronts for around £110.
So worst case it's £350, on a bike that you might be able to pick up for £2250? Still not expensive based on the fact that a good clean XChallenge with low- average mileage must be worth £3k anyway.
And you still have a set of "custom" yellow plastics to ebay afterwards (or respray at your leisure and then ebay on ).