Of course, I cannot be 100% sure but it's the diagnose in more than 99% of these cases.
I shall not be the one to judge other peoples skills and I don't know who looked at it for you, he may be right in his diagnose, but
it wouldn't be the first time we hear that "something unobtainable" needs replacing or "it would be too expensive to fix" when the
truth is the guy at the repairshop didn't know what or how to do. Servicing B&O is not like anything else.
Worth a drop of oil for a try, I think.
Replacing the arm with one from another deck requires a full alignment of the Beogram and that's neither easy nor quickly done.
There's a also a good chance that the donor mech. will have seized too, they all seem to do this sooner or later.
Martin