contrary to popular belief, you CAN turbocharge a flat4.
however you'd have to replace/upgrade at least the following items if you want to do it right:
ALL engine bearings
2nd oil cooler
without doing all this you will literally blow your engine apart or fry it. many people have thought about turboing an aircooled VW (including myself) but it's not as simple as it sounds. apart from the engine you'd have to upgrade everything else on the car because it's old and because everything on a VW beetle is designed to work on a stock car. any mod to one part will require upgrades to other parts.
yes i know many people have shown it done on the internet,... but it's not as easy as it seems. aircooled technology requires certain things to be done to make these engines work in NA form,... turboing a VW beetle is asking for trouble.
BTW you can build a powerplant for a VW beetle which can push output in excess of 200hp (4 times the power of a stock 1600cc vw),.. but it'll cost alot. my super was designed for 44hp (it is not the S model) and i've upgraded and changed EVERYTHING already. Soon i'll be upgrading the powerplant to produce 80hp (at least) but before that i still have to upgrade the front brakes, antiroll bars etc. now i know 80hp doesn't sound like much but do consider that with 40-ish hp my beetle is already as fast as a cultus (non injection) off the line and through the gears.
with VWs, the more you learn the more you realized how little you knew to start with.