A conversion that is/was available here in Europe was the Oettinger boxer/flat 6 which is an engine that VW was to introduce to the Vanagon, VW however dropped it when it became evident that they wouldn't have it finished until a year or so before the introduction of the Eurovan. So Oettinger bought/got the engines instead but I don't know if they make them anymore. These are watercooled 6 cylinder boxer engines that (I belive) have a lot of the parts coming from the ordinary 4 cylinder boxers.

A friend of mine in Germany has a Doublecab automatic with one of these in and he has made quite a few BMW M3 owners very surprised and very sad at the stoplights :-)

/Markus Samuelson 
Sweden