I did a search on www.hannants.co.uk and all i turned up were the weighted wheels mentioned by Jeff, and metal exhaust pipes. Of note were 3 conversions that were available. One turned your ordinary C-47 into an Li-2 (Soviet C-47/Douglas DC-3), another into the AC-47, and the last (and i think the most interesting) into a C-47C float plane conversion! This is the first time I've ever seen one of these!
here's a pic:
Sorry i couldn't be or any more help...