treatshq has a good carb on ebay for about $150.
If that is too rich for you, I'd still find and install a stock PA50II carb, then jet up/down as you need to. According to some other sites, the 12mm carb is described as somewhat oversized for the stock motor/setup. I can't confirm or deny these other statements, so accept them - or not.
Granted, you won't get the same output as from the 21mm carb that treatshq had, but it will work.