I don't know of any synthetic replacemants. You can still get the original cork float from you JohnnyRude dealer for about 3 bucks. I've heard painting it with model airplane dope will work. I just painted a new cork with polyurethane varnish. I understand the originals were painted with shellac, which just won't seal with today's ethanol blends.
Hammer, Old Johnnyrude carbs had cork floats. They appeared to be varnished. For a fuel tank I would think you could use some plastic wine bottle corks. Alternatively, you could coat the cork with some polyester resin.
