I'm sure the reg/rect items can be had elsewhere, but I'm not sure what good it would do you, unless you wanted to replace the stock BMW items with a single more modern alternative.
You could probably source the field windings and maybe also the rotor, but then you'd have to machine the rotor to fit the BMW shaft/seal. You would also have to manufacture the mounting frame for the windings to attach to the engine.
Overall I think the system is very reasonably priced (expect the price to go up in the not too distant future), when you consider the cost of components, machining and development.
The only area of potential weekness I can see with the system (like other higher wattage upgrades) is the additional weight of the rotor vs stock. I'll try to weigh all three (Stock/400/450) and post the info. But I think experience has shown that the weight increase hasn't been a problem for the front bearing.