:( :( Oh boy are you in for some fun !
My mate bought an mpi with knackered engine and superb chassis to replace his tdi with knackered chassis and gearbox, so have first hand experience of this one.
First the mpi has tons of electrickery that needs removing or the wires puzzling out.
The gearbox is totally different to a tdi, it also has a 1.4 transfer when you need a 1.22 for the tdi. You will also need the tdi rad and intercooler.
Make sure you take all the powersteering pipes off the tdi as well as the pump fittings are different on the mpi so pipes not interchangeable, steering box itself is fine.
Then you need to cut off the mpi engine mounts and weld on the Tdi ones, fairly easy to do.
Fuel lines no real problem as mpi has a return pipe, need to get the fuel sender assembly out of the tdi tank as mpi has a pump in it not needed.
We still are undecided whether the front springs need changing it seems to sit fine at the moment.
Biggest issue is the electrics, the rev counter is totally different but we are not too bothered about that. It's just there is a huge bunch of wires going to the ecu a lot of which relates to sensors etc so are irrelevant to the tdi.
Hope that doesn't put you off if you want more info PM me and I will see if I can help.