I can't imagine you'll fit a TD5 along with modified gearbox, fuel tank (you need the high pressure fuel pump, and if you have a standard crossmember the TD5 tank doesn't fit without mods), airbox, all the other gubbins and bits you have to adapt (power steering etc) cheaper than you could make a nice job of overhauling the Tdi, even if a little machining was needed. Plus long term, a nice 200tdi 90 will probably be worth more than a modified frankencar.
I gave my 200 a mini overhaul a while back, plus took some time to port the cylinder head & manifold, lapped the valves in, set tim