Spun a wheel hex inside the wheel....

[suicideneil]

What size hex, and plastic or metal (hex)?

I would say either jbweld or epoxy resin should do the trick , only issue being you have to make sure the hex is centered as to avoid wheel wobble. You will have to glue the hex to the wheel permanently though- might want to add some star-like fingers to the round hole before adding the glue though, so that the glue & hex 'plug' doesnt spin out the hole - like a loose pinion spins on a round shaft- a splined pinion and shaft doesnt have that problem.