I replaced as planned the rubber lines on the front brakes with stainless steel braided items, and braking became instantly, perfectly acceptable (at least, for my type of riding). I had planned to do it all along, but was suddenly overwhelmed by a sense of urgency when I squeezed the front brake handle while standing next to the bike, and saw the diameter of said rubber lines expand by several mm: they were actually the original, 45 years old items!
So Dezza (and I) were right, and once I apply the same treatment to the rear brake, stopping the old filly will be a piece of cake.
Thanks to Dezza and everyone else for their good advice; the lists of adaptable calipers and disks will come in handy whenever I decide to give my bike the resto-mod treatment (which I have done to both of my other vehicles from the '70s).
In response to Swiss Toni's request, here is a picture of the GS 1000, after it was spruced up (but not a nut and bolt restoration).