What is the best engine you could put on a classic volkswagen?

I want to put the best performing and fastest engine on my 1968 Volkswagen Fastback. Also would turbochargers, or other engine accessories help?|||Porsche, because the engines are fitted in the back of the car. So they are made to fit small spaces like where you would want an engine for your VW.|||See if you can find a 914 porsche engine with weber carbs. should be exact fit.|||haha i agree with porsche answer but just a thought i generaly put engines in cars not on them haha joking lol messing with ur wordings|||classic volkswagen what?|||subaru wrx 2.0 sti./ normally these engines are being fitted in the new beetle