It isn't too hard once you get the particulars down. Magura makes a 5kohm twist throttle that can replace the pot on a servo tester. Thats what I did for my ebikes. I am on my third bike build now.
belt driven? :P A bunch of guys over at a brewing forum I go on all have home motorized grain mills that use 12mm pulleys... shouldn't be too hard to find should it?