Hi,
The belt mod kit was from Rowan Engineering
http://rowanastronomy.com/productsa1.htm#heq5beltkit
There was no extra machining required and it maintains the same pulley ratio as the gears, so you can use the hand controller/EQUSB without any modifications.
I have not done any periodic error tests with and without the mod. I used the mount twice since i installed the belt mod, before receiving my NEQ6. In storage it has sat since.
cheers.