Hello Marko, The belt used on the spindle encoder is a 250XL037. Here is one I found on Ebay.....
250XL037 Timing Belt Goodyear Brand New | eBay
The numbers on the stepper motor belts are, Gates "PowerGrip" 80132 X 3/4. I have tried to find one , but, those numbers come up blank here...I believe them to be MXL belts with 132 teeth. The pitch is 2.03mm. You can find them, but, the widest width I found here in the states is 6.4mm. I did find a place in England that will make the belt...
https://www.beltingonline.com/index....0&cPath=40_252
Hope this helps