This is from memory from a very long time ago...
The tail drive shaft is piano wire. It is connected to the idler shaft at the front with a wheel collar with a special (dog stop) set screw. Loosen this set screw.
I believe that the tail rotor gear box is held in with a small hose clamp. Loosen the clamp and pull the gear box out of the tail boom a short distance (you may have to disconnect the long push rod to control the tail pitch). You will pull the gear box out just enough to allow you to get the belt over the shaft, then put it all back in place.
In some cases you may have to loosen the tail boom mounting screws to raise the boom slightly in order to get the new belt over the flange on the white plastic toothed pulley. The boom should be raised or lowered to get the straightest alignment for the drive wire.
The whole operation should take about 10 minutes.
If you fly a lot, you will break the drive shafts (again, they are just piano wire), so you will get used to doing this. You will get a lot of life out of a properly aligned shaft.
Good luck! I hope you enjoy flying the Cricket as much as I used to!