We've been breaking down a brand new MTA4 S50 for parts. There's still a few bits and bobs left, including the engine, the transmission and the wheels: http://www.ebay.co.uk/sch/choconutjoe01/m.html?_nkw=&_armrs=1&_ipg=&_from=
Everything is deliberately priced to undercut the competition, so if you're looking for any of these parts you can grab yourself a bargain!