The go-to solution back in the day was indeed go-kart wheels, which came with a load of standard drive components that mounted easily to each other - as long as you were happy using chains and avoiding punctures...You can always knock the central bearings out of commercially available wheels and make your own hubs, and in this day and age you'll probably require some degree of custom components no matter what you go for.

It's also worth noting 12" is pretty big if you're going for a low to the ground approach - standard go-kart front tyres are 10", and a lot of modern heavyweights are under 200mm for height...