If the engine you're planning to build is based on a late-1950s 30bhp type, then the later flywheel as shown won't fit the crank.
Your simplest option is to run the original 30-horse flywheel with a six volt starter (to match the flywheel) on 12 volts. Or get your 6v starter/solenoid rewound to suit 12v.
Yeah, that's what I'm going to do. Did it in the late eighties in my autocross days. Thanks for the reminder, KS.