You have not given enough information to answer all parts of your question: type of ignition key/switch, type of starter actuation (foot starter or electric solenoid starter), points ignition or electronic ignition(and type/brand). Changing to 12v and having an alternator right-away makes using all of an original wiring diagram very problematic.

Here is a diagram for wiring a one-wire alternator:
http://mightymo.org/public_html/images/Projects/DelcoOneWire/AmpMeter.jpg
from this website: http://mightymo.org/Proj_OneWire.html#Delco