Heres the build for the 49cc. http://motorbicycling.com/showthread.php?t=60263
These are nice little motors. Not much in the looks department, but if you want a reliable motorized bicycle this is a good motor to start with in a 2stroke.
If you put a common 3spd to drive on the left you have to drive the hub and use the original input sprocket as the output. Due to the power flow through the hub(at least the older Shimanos) you wind up with only 2spds(actually 3 but 2 are the same.
The reasoning behind the double left side drive was to eliminate all the parts needed to make the common shifter set up driven on the right. The one I have really works well. It requires careful frame and wheel set up, but once that is worked out the rest is pretty easy. Also due to the chain spacing bike chains are the only ones with enough clearance. I run HD BMX chains on all my builds and have never broken one. You just have to be careful when buying a rear sprocket not to get the thicker ones for the larger chains.