Wolfgang........Years ago, before VW offered alternators, I gutted a generator housing and stripped the armature down to just the center shaft, welded mounting brackets to the generator housing to accept a small Motorola alternator. found a double sheve pulley to fit the generator shaft. (two belts) Voila ! I had a VW with an alternator ! It worked fine for about two years and then came apart. On disassembly, I found that the long 9/16" shaft inside the generator was destroyed due to radial flexing as it turned. It wasn't worth making a new/bigger shaft because VW had now came out with an alternator.
Bruce