The main issue is that you could go tens of thousnads of miles on the cost differnce of swapping in the diesel vs building a normal air cooled VW engine, especialy considering diesel costs more. it might be cool, but...
The VW flat four is only 2 cylinders long, meaning there isn't much room for the length of an inline 4 back there. If you were to mount it transversely, you have to remember that the VW diesel (or really most FWD diesels) sit ahead of the front wheel centerline in their stock applications, so would sit ahead of the rear wheels in a Ghia, putting it in the passenger compartment with you. That woudl mean completely restructuring not only the inner body, but making an entire new frame and rear suspension.
If you do go with an inline diesel hooked up to a stock type transaxle, you should look into the engines in the Vanagon diesels. If you get a crashed diesel Vanagon and swap the entire drivetrain (the diesels use different gear ratios in the transaxles) you should have most parts necessary. You will have to fab up a radiator and cooling system, and of course, you need to make sure teh fuel system is ready for diesel. You're going to have to fab up motor mounts, as teh air cooled VW engine simply hangs off the transaxle, where the longer diesel need rearmost mounts (at the "front" of the engine, where the timing belt and accessory drive belts are).