Thanks Ewout. That plan is about all I have. Lukily it has turned out, that the transmission is ok. It was the flexible coupling connecting it to the shaft, that was torn apart. It still grips when I go forward because the prop pushes the shaft into the coupling, but in reverse it gets pulled back.
So now I am looking for a new flexible coupling instead.
Found a vetus part BULLFLEX830 but so far nobody in Vetus has been able to confirm that it matches the MS4A transmission or give me the exact flange dimensions of the coupling.
If anybody here has recently fitted a new coupling on a MS4A I would be happy to receive product details... Otherwise I might be ending up at volvo buying the real expensive stuff..