Andras,
As deanyel wrote, it could be just the internal wiring that is causing the problem so fixing that may get it running again. However, in my experience, the magnetic clutch fails (mine was burned out!) and that is a fairly common failure. Also the "REF POT" fails b/c it wears in spots and becomes intermittent.
There are other sources that are much cheaper than Beckmann so you could consider trying one of those. Prices are considerably less.
Pete Mahar S600 site (Uber Alles) has some detailed info on ETA repair. It is for a "dual" ETA car, a S600. The wiring is shown with detailed pics and it's worth a look.
http://w140repair.com/ETA_Rewire.htm