Entire books have been written on this subject. PC7 or PC11 epoxy putty is a good and durable material for this sort of repair work, but you can't just glom it into the existing cracks, or it'll fall to pieces again in short order. The cracks have to be cleaned out and given "tooth" for the putty to grab hold of, and you need to try to find the areas that are on the verge of cracking and go ahead and crack + prep them. If you do this yourself, it will take a lot more time and effort than you probably have in mind. Many, many hours. I've known people to chuck a wheel on a spare steering shaft and clamp it in a vise, then spend hours and hours on an as-time-is-available basis getting it ready, applying the putty, sanding it down to match. The priming and painting is the easy final step. The results can be quite nice indeed, but it really does take a whackload of work.
If your dollars are more easily available than your hours, talk to the Pearlcraft people in Australia. Here's a '62-'65 Valiant wheel after their restoration + pearlising work:
