With your van being an Auto, you are very limited to what engine you can replace your ill 2.1 with.
The auto only came with a 1.9 or 2.1 petrol engines and as the gearbox is a very different unit to the manual gearbox, you can't use the VW Audi 5 cylinder bellhousing.
Believe it or not, the cheapest engine replacement will probably be another 2.1 wasserboxer.
Any engine conversion would involve an adaptor plate, plus all the one off parts for the conversion.
I think the only other engine you might get parts to fit would be a subaru engine and look at
http://www.smallcar.com/ and see if they offer an auto conversion kit.
It will cost more than buying a new 2.1 though.
Good luck.