Starter will need to remain 6v, otherwise the starter will rotate too slow to start engine and will overheat. Most starters within a manufacturer (e.g. GM) have similar bolt patterns and drive gears and can usually be swapped.
Ignition is not a problem until you get to modern electronic systems.
Other engine areas to consider are instrument sensors (coolant temperature, oil pressure,) and actuators (electric choke). Most sensors can be replaced with 6v sensors, choke can be changed to manual.
