Hi Lars,
I purchased belts and pulleys from
Sdp-Si, but it was quite some time ago and today there are cheaper options (BangGood, ebay...). GT2,
60 and
12 teeth (5:1).
It was a tight fit, some machining was necessary, it was pretty fiddly to place belt onto motor pulley, but it is perfectly doable with adequate patience.
Motor (similar) came from
ebay.
There is even a
set 4:1, not sure if bellt length is adequate, it mat be specified... if not you can buy belt of any length on BangGood for ~$5.
As to your existing controller, 400s/rev motors may not work, the problem will be overall mechanical reduction ratio of steps/arcsec.
You will have to find the right pulleys as well - what kind of reduction you have in your mount?
Is it 12 teeth gear on motor, 47 teeth gear on worm (idler (36 teeth) is not relevant for reduction ratio)? It that case, you will have to find timing pulleys with the same number of teeth (12 is OK, 47 teeth pulley may prove to be tricky... however you can print one with 3D printer) - if you want to keep the same controller.
Or, if it is possible to re-program controller for different mechanical reduction ratio, you can use common pulleys.
My EQ6 original controller was old-style (only tracking and fine positioning) so I went straight to Bartels system, and later to OnSTEP (Arduino platform), using 16 ustepping pulse/dir Pololu drivers.