With many thanks to Philippe I received a proposal for a
engine swap to a 1.9L TDI. This is a design by Winkler:

You will need a 70 PS Turbo Diesel Vanagon and a TDI
engine with accelerator pedal, wiring, computer, catalytic
converter. A converting kit will cost DM 7,900.00 (engine
included) and  the optional oil cooler DM 790.00 which is
recommended. Labor will costs DM 12,500.00 and includes
engine and other additional parts. German drivers will
need TUV approval which costs an extra of DM 650.00

It is also possible to convert a Wasserboxer. Than you
will need some parts from the TD engine:
 - Bellhousing from a 50 PS or 70 PS Diesel
 - Starter from a Diesel
 - Oil pan from a Diesel, Turbo Diesel recommended
 - Oil pipe which goes into the pan
 - Flywheel from a Turbo Diesel
 - Exhaust system from a Diesel
 - Rubber engine support from a Diesel
 - Engine support from a Diesel, have to be modified

The small pictures I found in the catalog are copies
from a regular service manual and cannot be used for
a redesign of the engine support. But I think this
task is not very difficult. The TDI engine are available
as 90 PS or 110 PS. Last one is a chip-tuned engine I
guess.

Helmut
'80 Camper