The stock solution works fine, but upon tightening the belt the motor bracket skews a little causing the belt to rub against the side of the idler and drive gear. To prevent that I made a replacement, which does the job nicely.
Along side the motor mount I also made the belt clamp, which doesn't bend the belt upward and a stronger belt tightener.
For mounting the stepper you will need longer machine screws
2x M3x12mm and
Which should have been included in the kit.
After you print the belt tightener you will need to drill out the hole and file it a bit.