Depends what you are after. But it's a fairly simple build. Google a 4x4 is Born. A series where they build one.
Essentially strip the body off. Remove the rear cross member and weld on a Defender one. And weld on Defender outriggers to mount a Defender body. You'll need to sort out a fuel tank. Either like a Tdi Defender or in the bed. And adapt the transmission tunnel to fit over a Disco driveline.
You'll need to source some Defender body parts. As for legalities. The chassis has been welded. But it might be seen as a just a body change. So may not need an IVA. I don't think there are any concrete guidelines. And largely down to interpretation.
There are other ways. You could make it look like a Series 1 and fab up a bulkhead and panels. Pretty easy and is what most CCV trials vehicles do.
Or as another option there is the Sahara kit. Which makes a Defender looking vehicle.