I'm afraid it's not an off-the-shelf size like the ones, I've seen sellers on Ebay provide very cheaply.
I've replaced a couple of them already and also replaced a motor or two with badly worn bearings on
Beograms coming in for repairs here.
Some of the belts are simply too small, pulling the subchassis enough towards the motor to see
the subchassis rub against the main chassis with lots of rumble etc. as a result (plus stress to
the motor bearings).
Other belts seem to have almost no elasticity with lack of free movement of the subchassis as the result.
If the subchassis swings a little, the belt either slips on the motor pulley or tightens too much, restricting
the damping.
Maybe not quite as cheap as the generic ones (that doesn't fit) but I can supply correct belts, custom
produced to size etc.
Available directly from me and also on Bonanza.
Neopren, kautschuk and sometimes nitrile in a certain mix will give the correct elasticity and friction (which
none of the standard belts will provide anyways).
Martin