You can get the full gamut of Brompton spares - you could actually build a whole bike in kit form from the range of spares available at St John Street cycles. I've got a few 'real' Brompton parts for the Merc I'm souping up from there and have not encountered any problems with any parts not fitting as yet - from the rear triangle to the forks and seatpost.
http://www.sjscycles.co.uk/category-...oulton-766.htm