The most cost effective swap is an AHU-code 1.9 TDI. Easiest to swap using
the pump from the 2.8L 4-cyl TDI in the LT, also available in Germany.
You'll double the power in stock form, more with larger injectors. Of course
no matter what you choose, the trans will need upgraded 3rd-4th gears if not
a taller final drive.