running backward is cause by too little momentum to get the prop over top dead centre at idle. Momentum can be increased by higher rpm, or higher prop (flywheel) mass, or both. Wheel chocks come to my mind as an option.
It also may help to retard the ignition somewhat. If one of the ignition sensor screws is removed, the ignition can be adjusted quite substantially by rotating the sensor with the remaining screw as pivot point.
After starting, the ignition will fire at about top dead center, but will advance after a fixed number of revolutions. At the low idle we need to keep the plane from moving, the engine may then kick back and run backwards. I found the engine can run with retarded ignition quite well, and without noticeable power loss.