15 or so years ago I built a a York onboard air system for a Jeep I had at the time. The Jeep had a serpentine belt but I found the easiest way to drive it was to use a second V belt off the alternator. I took the serpentine pulley off the alternator and found a small V belt pulley that fit really good against it at a wrecker. I had a machine shop weld it up dead center and balance it. I built the compressor mount so the compressor and drive pulleys lined up correctly. You'll also need to construct it so the mount has a belt tension adjustment. I made slots for the mounting bolts. This was a bit of work to make it fit real nice and not look like crap. Now I would probably just buy a kit if one was out there.
This company is a great source for parts to make your own setup. Forget that they are made for Jeeps. The Jeeps in the 90's were using Sanden compressors I would almost think one of these kits would be a good starting point. I don't have AC now so I have no idea on the location of the AC compressor in a Defender so I might be wasting your time. Take a look at the mounting instructions.