Hi I am thinking about changing my 2.5TD engine in my 90 to a 200tdi disco engine. what would I need to do make the engine fit/work pipes etc.

Have a good luck around the site. there are some great threads about this....Thousands in fact. Its real easy, 1 out 1 in for the engine and a bit a fookin about with pipe work. A lot deepens if you just have the donor engine or as I did get a whole 200tdi discovery. My way meant I had all the pipes and stuff I needed between the 2 landys