Its a very simple hub- like this one:
http://cgi.ebay.de/SACHS-NABE-Hinterrad ... 5d2992343e
the bigger sprocket I got from a Moped shop- my is a 25 tooth one ,it has to fit on the Hub-its the same size BMX Hubs have for the sprocket- but I had to cut the Sprocket a little more slim, because it...