I made the motor mountings to connect the NEMA 17 steppers to the existing out of a piece of aluminum using only a table saw, drill press, and an inkjet printer. I drew up the adapter schematic in FreeCad and then (2D) printed it to scale and glued it to the aluminum plates. Then, a machinist punch was used to create a dimple in the aluminum so that the holes could be accurately drilled on the drill press.
After a lot of search on ebay, I found some suitable motor couplings to connect the stepper motor shaft to the worm. The trick here was finding couplings that had an outer diameter less than 18 mm (I think) since there is limited clearance on the mounting plate that holds the worm blocks. Most couplings for shafts of this size have an OD of closer to 25mm. This wouldn't be a problem if you have access to a lathe, though.
I designed and printed some friction fit covers for the motors that the DIN connector could be screwed to so the stepper wires couldn't get yanked around. This works pretty well, but I've found that the long stepper motors can torque the 2 screw motor mount on the G-11 enough to flex quite a bit.
This is an awesome project. BTW if you used pla to print and have trouble with the plastic shrinking or falling apart, try using petg, it works incomparably better for cold/hot/humid weather conditions.