It's not impossible to rebuild a starter with components, as said. Bearings, brushes etc tend to be "a size" that is also fitted to a hundred other things, rather than bespoke.
For reference, I sourced a new voltage regulator for our Mk2 Polo, for about a tenner, compared to the £x00 for a full alternator.
Try giving ElectroDiesel a call, they were the only people that could find said regulator - properly old fashioned place but they may post things to you if you ask.
(Failing that, if they can get the bits, I'll happily collect them and post them to you, they're relatively local to me)