The conversion is really easy, assuming no electronic devices have been added.
Disconnect the battery, swap the connections at the ammeter and coil, reconnect the battery with the opposite polarity, and polarize the generator. That's all.
Probably the easiest way to polarize the generator is to pull the wire off the 'F' terminal at the control box, and touch it to the 'A' or 'A1' terminal momentarily. Some small sparks are normal. Then connect it back to the 'F' terminal and you should be good to go.