A old square shaft wheel can be converted to a keyed/taper shaft wheel.
Some options .. Thanks Ray !
There’s a few options: you could just use a different wheel entirely. Detmar made the later style wheels and made several hub styles, but they all relied on a 3 screw system to hold the spikes to the hub. Some hubs had interchangeable adapters. A little use of some plate and you can modify your old wheel to use the new style adapter. Lots of possibilities.
The adapter you posted (first picture) above is what I would do if I didn’t have all these parts. I’d place that inside of the old plastic hub, drill and tap where needed, and the only other mod you would have to do is make the G center cap removable. Adhesive backed strong magnets would be perfect.