The mechanism belts can still be purchased, for example at www.csi-elektronik.de. Belt symbols are BLT17143 and BLT17334.
The most popular failure of the mechanism is that it stops properly switch between play and record modes. At the back of the mechanism there is a pusher that pushes a switch on the nearby board (see red arrows below). The pusher should push the switch only when recording and not when playing. The failure is that it pushes the switch during playing too. The reason for the failure is a thickened grease below the pusher that makes a white plastic lever stick to the pusher (see green circle below). It should be best to dismantle the mechanism and clean it. I managed to clean it using spray dissolvents like IPA without taking the mechanism apart, but it may be just not so thorough and long lasting.
Thanks to the Robotrontechnik forum’s user Guido Grohmann for helping me figure that all out.
Another issue is with a plastic transparent cover of the cradle where the tape is placed. It has a handle that latches when the cover is closed. The handle can easily brake, because with a tape inside and a malfunction of the mechanism, like loose engine belt, it is not possible to unlock the latch and it blocks opening the tape recorder’s cover. I managed to fix the handle with glueing a piece of metal extension to it using cyanoacrylate super-glue.