Well, the coaster hubs generally will fit with the only modification being you have to remove or grind down the bearing cap a little. On the three speeds, the whole deal is about the same size as the center.

Better get cozy with a file/dremel/torch, there's a lot of material to remove from the sprocket if you want to use a three speed.

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If it ain't broke, and you mess with it long enough, it will be.