Bring the piston up to the top of the stroke for reference, and leave it.

Remove the backplate without loosing its rotor orientation to the crankpin. Be careful and look for where the opening is, because just the slightest bump and gravity will make it turn. Take note of its position, or what rotor slot it was originally in.

There are two slots in the rotor disk, one each side of the brass axle pin.

Rotate the disk 180 degrees and re-insert the backplate, being careful the crankpin returns INSIDE the slot.

Looking from the rear view of the motor, the carb opening has to be turned 90 degrees to the right.

The rotor timing will still be off slightly, but the engine will work in the opposite direction.

That should do it.

If you loose reference between the drum rotor and the crankpin, start over with the carb mount towards the top, and the piston on its way up, and the rotor just begining to open.