The majority of Matts customers have been for recumbents, so the drive was located behind the seat. It was brilliant the way he designed the shaft joint, so the drive could be re-configured into several shapes (an "L", a "V", an "I", etc). I'm a big fan of Matts work.
My difficulty in finding a 20mm wide pulley with a 10mm bore, that has a reasonably low tooth-count, leads me to believe that's why GNG chose to source a custom motor shaft with the 14T built-in (non-removable).
If you are willing to risk paying up front for 100 pieces, there are plenty of shops in China that will gladly make any part you want (regardless of potential copyright issues), and they will make those parts to any level of quality you specify, from dirt cheap to high grade.