Its been asked before and firstly its worth emphasising the difference between 1.9TD and 1.9TDI - the latter is a later engine which uses an ECU instead of mechanical fuel injection, so it would need the ECU and wiring transferring too. Depending on the age of the donor car, it might have issues such as immobiliser, etc in the ECU which you'd need to sort out. Also I believe the engine mounts from the stick big block golf, are bolt on (but unsure). If not, then you're into fabricating your own mounts. Also check out the gearbox - I think TDIs came with an 02A transmission, I suppose you could use a high ratio (the diesel would need to be) 020 box from a Mk1 or Mk2 Golf diesel. Otherwise again, you're into fabricating mounts although in this case you can buy conversion items. Also if you use the 02A you'd need to sort out hydraulic clutch and cable change, etc.
Basically, once you get into the details, the amount of work to undertake is equivalent to a 1.8T or VR6 engine conversion.
But with sympathetic tuning, would make worthwhile power.
