There are some auctions on eBay for these:
I get mine for the same price from Mike @ Simsonmotorbikes.com. You need the 48-60cc version. It has a smaller jet than the 70-80cc carbs. They mount right on with a little tephlon tape (despite some tilt). Check my photo in the post above. I wouldn't be opposed to trying a Bing or Delorto carb if you have one lying around.