Alternative engine 1.9TD

Turbos

Re-indexing, see Diesel Engines Turbo (Re-Indexing)

Inlet Manifold

The AAZ inlet ports are 'D' shaped, the JX are oval, which match at the top not at the bottom. You could use the JX manifold 'as is' and accept some reduction in cylinder filling. The is scarcely enough material to open the ports out, as this becomes too thin to provide a wide enough mating face for the gasket. Opening the JX manifold out fully requires building up the edges to support the gasket and the resurfacing with a surface plate (See Cleaning/Trueing mating faces)

Using the AAZ manifold is probably the best option, but this has the inlet stub on the top, not the outside face. A different inlet hose and general arrangement is then needed. In theory it wiould be possible to adapt the AAZ manifold to the side mounted inlet stub, as the casting is nominally the same.

Exhaust Manifold/Pipe/Silencer

Heat shields...

Manifolds/Heat Shields

Exahaust pipe

Whilst the std JX pipe (2.25") is sufficient, a 2.5" system throughout with less resistive silencer should be of benefit if increasing the boost and fuelling (to keep exhaust manifold pressures down). (Generally reckoned to produce more torque, example of custom system to follow)

Silencer, custom heat shield Silencer - custom heat-shield

Oil Quantity/Dipstick Mods

Oil quantity on all Diesel kit mounted (52 degree angle) engines can be critical, too much being the real problem 
4.5 litres is about max. (without allowance for an additional oil cooler, a good addition to these installations) 

HarryMann: Also note that with some diesel dipsticks the correct oil level won't even be on the bottom of the stick, so shortening the dipstick tube is the normal remedy (for the AAZ anyway). About an inch and a quarter reduction from memory, or where there's a natural change in section, so that the yellow plastic scroll-clip can be re-fitted, finishing it all nicely.

Correct oil level is then about minimum on stick, but calibrate it anyway, with 4.5 litres + allowance for oil cooler if fitted (another 0.5 ~ 0.75 litres depending on cooler size and pipe run)

Injection pump Mods

Ther are a couple of mods that tend to go alongside raising boost pressure to increase performance. Whilst simply increasing fuelling on the main fuelling screw achieves a result, this is a bit crude, and does not balance the additional fuel across the range with boost levels. Adjusting the LDA pin or changing it for a re-shaped one can be done relatively easily in most cases. IF you require more revs, to get above the 4,200~4500 governor limit, this mod can also be done, though is not for the feint hearted!

VW GTD forum's Governor Mod FAQ