You'll either have to find one of the aftermarket ones that was made to replace the cush drive system, or make an adapter to use a sprocket for another machine. The aftermarket ones aren't available new anymore, only NOS.
The custom sprockets someone already mentioned are aluminum. Sprocket Specialist only make aluminum ones anymore also, so no luck there. Those don't last nearly as long as a steel one, and still cost a lot more.
Whatever you bolt on there, has to have a slight offset, or dish, so it's placed over the center like the original one.
I gave one of the (Sunstar?) aftermarket ones away last year. I'll see if I can find the thread, because there's a picture of it in there.
The story of three wheels and a man...