We don't have a "neighborhood" BMW motorcycle shop. The nearest BMW motorcycle dealership is more than 4 hours drive from me, so dealing with them would still be mail order, and a crap shoot since I don't have any idea of their ability to supply correct parts for older airheads. Kinda like throwing a dart at a map and going with whoever is closest to that.
A recommendation of a trusted, well known dealer or online vendor is what I am looking for.

Thanks Scot. That is helpful information.

The typical failure of these master cylinders is from "not working great" or "failure to bleed properly" rather than outright leakage. If your rebuild fits the former rather than the later reason, this is usually caused by the long-term combination of water and old DOT 3/4 fluid.

If you'll look down the bore you'll probably see a tan varnish-like coating on the bore and evidence of caking at certain places. This can easily be removed by wrapping a 1x3" piece of 400 grit onto a 5/16" dowel and using a drill motor to hone the bore.

It's also highly likely you're going to find this same crusty stuff inside your caliper too. Best to take it apart once.

Wash everything out with hot soapy water and allow to thoroughly dry. A hair dryer helps. Coat the bore and new seals with new fluid before re-assembly. Hope this helps.

Thanks Wobbly.
I did hone the bore on the M/C yesterday, but was still concerned by the remaining evidence of pits (obvious water corrosion), so today I found a new replacement M/C in stock at a So. Cal. BMW dealership and ordered it. Only twice the cost of the piston w/seals, for an entirely new unit.
I'll be dis-assembling the calipers tomorrow, and hopefully only needing new seals for them.

A common misconception. The bore doesn't need to be in pristine condition to work just fine. If you'll make some measurements you'll find that the pitting is where the seals spent 99.9% of their life... at the un-activated position. The pitting would need to be at the compressed position in order to affect braking pressure.

Never the less, the main thing to come away with is the knowledge that not changing the brake fluid on a regular basis has a cost in denaro and downtime.

What did you decide on the hoses ?

The hoses look good, flexible and have no surface cracking, so I am leaving them alone for now. I plan to replace them this winter when I do some other off-season maintenance/repairs.

Mine had more "green, scaly, embedded crustaceous growth" than "tan varnish like" coating.
More akin to removing barnacles from stone than polishing a bore.
I'll send it to you if you're into living dangerously, but seriously, it was in bad condition.
I worked in the M/C business for many years, and we never tried to re-use an aluminum m/c that was so badly corroded internally.
Besides, the difference in cost between a $70 piston with seals and a $170 complete replacement master cylinder, with new reservoir, cap, rubber seals, screws, etc., is a no-brainer to me. If I purchased the former, and had any seepage, or poor braking pressure, I would have lamented that purchase.
As things stand, I installed the brand-new master cylinder today, bled the brakes, passed by and greeted the mailman who delivered the m/c when test-driving the bike, then later on went for a great late evening back-country-road ride, testing and trusting the front brakes all the while. Stopped for an excellent burger and beer at the halfway point, too! I did discover a slight front end side-to-side wobble at about 100MPH on the way home, though. . . . . perhaps another "winter project" . . .