Engine Conversions of VW Campers

[andoverben]

Mrs Andoverben is Preggers and I have developed this sudden urge to go and buy a VW Campervan and drive across Europe with Wife and Sprog. The Urge itself has shocked me however I have told family and friends they have permission to shoot me if I start looking at Caravans ( my wife said she would divorce me if I came home with a caravan anyway)

I don't however want to sit at 60 mph on the motorway and want a bit more grunt, so that got me thinking as to slotting a different engine in.

So Far my thinking has either been VAG TDI Engine to keep it in the family and have a bit of economy as well, or to keep the authentic sound a Subaru Flat 4 such as out of a Legacy. Obviously both of these are watercooled which requires some thought in cooling them.

Does anybody have Experience/Suggestions as to what engine options fit and what has been done before - My pockets won't strech to an 911

[andoverben]

Some great advice so far and am glad to hear the Scoob Flat 4 has been done.
Just to clarify I am not looking to do 120 or put a Porsche Engine in I want a 70's Style Bus that can sit at 80 - 90 and cruise happily