Simply giving these Diesels more air does not increase power output. The ALDA is the anaeroid (sp?) compensator that instructs the injection pump to provide more fuel under boost.
One thing I forgot to mention was to hook up a boost gauge to one of the nipples on the intake manifold. If you see boost, then the problem is not with the turbo. You'll have to test this with a load on the engine such as while driving the car. Reving the engine in neutral will not create boost.
I believe the turbo can be rebuilt and obviously it can be replaced.