Hi it is fairly easy to upgrade the engines on defenders due to the large amount of space in the engine bay. I have seen a 6.2 liter GMC diesel engine in one, and all that is needed is a conversion plate to mate it to the gearbox bell housing, there was one of these engines on ebay last month which sold for $1000.00 I also have a friend with a mercedes 3 litre 6 cylinder engine with the conversion plate which should drop straight in the defender very torquey, and good cruising speeds of 110mph!!
The engine I have just put in mine is a 200tdi from a newer defender, which has made a very surprising performance enhancement.
I also know somebody who has fitted a daimler V8 engine and also attached a supercharger, this thing beats subaru's and evo's from a standing start, and is one of the only trucks I have seen at the quarter mile drag strip sprint!!