If you're talking an older air cooled Beetle, then you ned to talk to Bernie Bergmann Enterprises, as they make a LOT of different turbo engine combinations for those cars, as well as turbo kits.
But to be honest, you don't need a turbo engine to go fast. Just build up a 2110 or 2180cc N/A bug motor. Relatively cheap and capable of propelling a Bug to 11-13 second quarter mile times, depending on buildup. Even a 13 second street Bug feels like a rocketship.
If you do that, don't forget to upgrade the trans, as well. Companies like TransForm have warrantied high performance transaxles for failry low prices, and you'll need the SuperDiff option to keep from blowing up the transaxle with a big Bug engine.
The nice thing about a bug engine is you don't need to create a cooling system and route hoses up to a front radiator. It's just a bolt in thing.
I'd stay away from using an early Porsche engine. The 356 stuff is almost identical to a Bug, but it costs vastly more and has less potential. The 911 stuff is pricey and heavy, even the early 2 liter engines, in comparison to a bug engine, and it sticks farther back (which isn't what you want in a Bug). The only "Porsche" engine really to consider in a bug swap is the VW Porsche Type 4, as found in the 914 and 912E (and the VW 411/412 and later Type II Busses). It'll bolt in, but you need to change the cooling tin to fit. They start out larger, however, with a 2 liter avaialble. You can start with a 1.8 or 2 liter Type 4 and build up to 2.7 liters in streetable form (and over 3 liters in race form). It's more expensive than a Type 1 Bug engine, but it's the way to go to get 10 second N/A street Bug times.
Go pick up a copy of Hot VWs magazine and start perusing the advertisers. You'll find more info and cool ideas than you ever thought possible.