I am still in Texas and do not have my manual, but as near as I can remember, the diode is a 6volt zener to protect the bulbs from over voltage. The 12 volt system is only capable of putting out 12v as long as all the bulbs are working. Disconnect the dioide and replace all the bulbs with the correct 12 volt and you should be ok. The horn may be unhappy at 12 volts but unless you lean on the button I don't think it will burn out. It may make your brake lamp really dim.
The ignition uses a separate coil in the mag and is most likely the same in 6 and 12v peds. Check one of the ped shops and see if they both use the same replacement coil.