The only way to upgrade diesel Vanagons, to my knowledge, is to either:
1. Put a 1.6l TD in the 1.6l NA diesel's place
2. Put a 1.9l NA diesel in the 1.6l NA diesel's place
3. Put a 1.9l TD in the 1.6l NA diesel's place
4. Put a 1.9l TDI in the 1.6l NA diesel's place
5. Drop a 4 cylinder Fuel Injected engine (gas) in the diesel's place.
FYI I sell new 1.9l TD engines with the heads setup to recieve the 1.6l intake
for $1650.  This allows you to either drop the motor in and use it as a NA
motor or to slap the 1.6l turbo setup off of a Jetta or Quantum on there and
have a Turbo diesel.
I also sell the 1.9l turbo injection pumps now, new for $550.
Hope this help you.
Ken Wilford
Van-Again
John 3:16