If you have a sachs that has the pedals attached to the engine case, you don't have a rear break cable. There should be, on the right side of the motor, a lever that is on the pedal shaft. Rotate that lever it untill it's pointing down, there is a rod that goes between the lever on the pedal shaft and the lever on the rear wheel that works the brakes. You pedal back, coaster brake style for the rear brake. You should be able to rig up something using threaded rod and a few bolts.
On some Sachs mopeds the lever on the handlebar works the decompression and starter clutch.