[Daves55Sedan]

Go to your favorite chain auto-parts store and look in the shelving for tubes of goop for modern car bumper crack repair. That liquid plastic in those tubes is about the closest match to the "bowling-ball" grade plastics moulded onto steering wheels of the '50's.
You will hear other people recommending JB-weld or bondo or some other stuff, and that can be made to look nice initially, the dissimilar materials expand and contract at different rates causing cracks all over again and the repair to fall out.
I don't remember the name of the stuff I used (bumper plastic repair), but it holds up real well and if you usually store your car in the garage, your repair will most likely last past your lifetime.
Lemme put it this way. I used this stuff for crack repairs on two '55 Ford steering wheels, both of which sat outside in blazing sun and freezing cold for decades and the repair has still held up well.