We had a kersten with one gulley brush, for a replacement brush they wanted over £120
I created simple linkage and had a brush made to measure cost just over £18
Not surprised they have not got back to you thats the same attitude we experienced.
Reason we use Billy goats in essence very simple design, As Carl says that machine is simple to create having watched the video again, personally I would prefer a support at the front (castor) in order to allow an exact height to be set, otherwise its down to the operator to determine the load on the brushes which could make them wear quicker.
We have a local company which have helped us in the past ref Billies, I will ring them 2mw and see what they recommend / say,
Mike