Do you really mean making a petrol engine run on diesel? Well let's see, you'd need a new head (with the right sized holes for injectors rather than spark plugs), the injectors and pump, new pistons, a new crank to give you the higher compression ratio....
I suppose if you are the kind of person who walks around with one arm tied behnd your back becasue you like a challenge it might be possible. And the block might even surivive for a few hundred miles. But why oh why oh why???
If on the other hand you mean can I swap the petrol engine for a diesel, that is fairly straightforward - assuming you mean a Land Rover diesel, it's just a bit of wiring and plumbing to sort out. Easy enough!