Since there seems to be no information out there (just a bunch of loose ends in this forum), I figured I'd share my experiences with overhauling the cassette mechanism on my Beosound Century. And, more importantly, the parts list.

Belt: Flat belt, 4mm wide, my old belt had a diameter of 71,6mm, ordered a 70.0 mm x 4.0 mm x 0.55 mm belt, works fine.

Pinch rollers: 9,8mm diameter, width of 6,3mm for the rubber part, 7,3mm for the plastic part. The bolt is 2mm, however the hole is a press fit and more likely 1,95mm. I got 10.5mm x 7.2mm x 2.0mm (diameter x plastic width x bolt hole) pinch rollers as a replacement. Tried to glue down the bolt, as apparently the design of the deck relies on the bolt being fixed, and it slipped in the 2mm hole - tape speed was still uneven, so I ended up just pulling the rubber from the new rollers and the old rollers, and put the new rubber on the old plastic. Perfect fit, works fine.

Gears: Mine were still working somewhat, but had missing teeth and made that awful squeaking / clicking sound every now and then, especially after autoreversing. After lots of cross-referencing it turns out that the tape mechanism used is identical to the one in the Philips DCC600 - there's a person on eBay in Poland that sells reproduction gears for these, they are a perfect fit for the Century and work wonderfully.

All in all, that was around 40€ in parts, and my Century works great again for listening to terrible mixtapes.