after you start going too much above 1600 (which is the block most of these engines are built on I believe) you start to lost reliability and life. I was told when I had my 71 superbeetle, that 1776 was about as big as you could go without risking too much with the engine. You can get an engine punched out to 2 litres, but I wouldn't use it for anything but drag racing. remember these are air/oil cooled engines, so you punch them out too much, the heat increases and they can't really cool themselves well enough. You can get Gobs of power out of a well built 1600 if you build it right.
just my 2 cents.