GruBee Skyhawk T Belt drive 4G transmission kits and service parts for motorizing standard bikes and also Multi Speed Shift for Gru-Bee Gt2A frames with built in Gas tanks.
www.grubee.net
double-check to be sure that you have a tapered or straight shaft, then order the right one.
1A or 1B SHOULD be the right ones (depending on what reduction ratio you want)