I finally got around to it:
The original belt is round, a thickness diameter of 1,5 mm and a belt diameter smaller than 35 mm. A new original depending on elasticity(the old one has very little left), likely measures between 30 to 33 mm diameter. The closest round one I found locally was a round 1,2 mm thickness 35 mm. Purely on better friction I hoped it may save me, but no.
The next closest was a square 1,5 mm, 32,5 mm diameter. I was worried it might start twisting immediately, but after some testing it proved flawless.
Tension is nothing special and may well be less than an original belt at 32,5 mm, so no the motor wear does not worry me.
It may not last another 30 years, if this really was the frst belt, but for €1,02 (I assume I can´t tell where I bought it?) I´m not complaining.
Happy listening.