This is what you need:
https://www.beoparts-shop.com/product/belt-set-3/
Changing the flat belt is a bit challenging.
You need to remove the steel frame. A couple of screws on the side.
Although you don't have to, the easiest thing to do next is to remove the large circuit board. You need to desolder the circuit board from the tape mechanism. There are two solenoids, one on the left and the right side of the mechanism. Each have two pins. Desoldering the pins will allow the pcb to come loose. This gives you more space to access the belt.
Looks like your belt has melted. Use isopropyl alcohol to remove the belt residue.
Then a drop of oil on the spindles/capstans and you should be good.