Merc exhausts last a long time but inevitably go eventually. I would be more concerned with the catalytic converter/s altho some folks have "gutted " the housings and welded them back up and still passed the MOT. You should be able to source pattern exhaust parts from Euro car parts or GSF. Failing that there's always the st/st custom exhaust shops who will make one up to your specs. Since its a twin system just make sure you are sitting down when you hear the price.