Yes, several people have changed synchros from gas -> TDI.
You need a TDI diesel engine + electronics. Then motor  mounts,
 oilpan and some misc parts from a Vanagon Diesel. Then you
need a special synchro diesel fuel tank from Germany.

On the vanagon list and list archives (gerry.vanagon.com)
you will find lots of detail advice.

Best,
Martin