What alternative carburetors are available for a VW Beetle 1600?

There are a lot of different ones. It depends on what you are looking for. Do you want a setup that is good for daily driving, something that is great for the dragstrip, or in between? For Daily drivers, there is the 2 barrel Progressive Weber center-mount carb. It has a primary barrel for normal driving and a 2nd barrel for when you need to open the throttle. There is also the dual single barrel carbs setup, like the dual Weber ICT's, which gives you more power, but won't break you in fuel consumption. For racing, there are the dual Weber 48 IDA's. These can be bored out to about 51.5. This setup is not practical for daily driving, but if properly setup and tuned, will be pretty quick. If you're going with this setup, though, I'd recommend getting an engine with a larger displacement, like a 1776cc or higher. For something in between, there is the Dual Weber 40 or 44 IDF's. It will give you some get up and go without being too terrible on gas. It all depends on how mild or wild you want to get (and how fat your wallet is!). The 1600 dual port is a very versatile engine that can be modified in a lot of different ways, depending on what is important to you.