if you are in the t4 section of the t4 forum click on the engine section then at the top theres a 1.9tdi information library post at the top. All of the completed build threads and documentation is in there. Aggies original thread is a good place to start.
Thats where ive found all the info to do mine,
To summarise converting a 1.9td (abl) into a 1.9tdi you need to
*obtain a 1z or ahu engine from a donor vehicle as well as the engine loom, ecu, immobilizer electronics and chips from the keys
*Purchase new flyby wire accelerator pedal and bracket for an early 2.5tdi from vw.
*swap over your sump,pickup and turbo/inlet/exhaust manifolds from the abl engine
*drill a new hole for your abl dipstick in the blank already in the back of the block and fit your abl dipstick
*blank off old dipstick hole
*drill 2off M8 holes out to 8.5mm and tap with an m10 thread to take drivers side engine mount
*drill 1off M6 hole to 8.5mm and tap with an m10 thread to take drivers side engine mount
*swap over all your other t4 ancillaries such as power steering pump, alternator etc
*Source a 2.5tdi airbox lid
thats pretty much it really, also worth doing the following things whilst the engine is out.
*replacing cambelt, waterpump, auxillary belt, power steering pump, sump gasket, rocker cover gasket, oil seals (if they are leaking)
*fitting a new clutch, the golf flywheel is actually bigger than the t4 one and is compatible with the old t4 gearbox you are reusing. You can either fit a golf clutch or a 2.9 vr6 corrado clutch to it.
*The fifth gear from the golf gearbox can be used in the t4 gearbox to give you a taller 5th gear, reduce engine stress on the motorway, improve mpg and reduce revs.
Im looking to go for more power than the standard engine gives too so have obtained an uprated intercooler (sprinter/lt intercooler) which i plan to fit at the same time as the conversion as well as a decat pipe and then once everything is all running smoothly buy big injectors and off she goes for a remap.
this is the easiest way to do the swap, not the cheapest but in my opinion id rather do it properly, sure you can knock up a bracket and faf around with your existing pedal to save money but for the cost of vw parts designed to mount these exact parts properly in the vehicle you are using why bother??
You could also use the standard intercooler from the donor vehicle no problems.
The airbox lid again allows the golf maf sensor assembly to fit onto your standard t4 airbox base properly, the existing lid can be bodged but why bother?? OEM look is the way for me.
You can also use the golf turbo and manifolds if you wished but you would then need to fabricate a new downpipe. Some people have preferred this route, but the turbos used are either exactly the same anyway or are equivalent in performance so why bother.
any questions feel free to ask, i clued myself up pretty well before biting the bullet and buying everything, im currently in the situation where ive replaced all the bits required,did my 5th gear swap 18months ago so one less thing to do, prepped the loom down to whats needed ready to go in the van, and all the rest of the parts are sat in a big pile awaiting removing my abl engine on Friday. Game on!!