Rathlindri - that's a very pretty pair of bikes you have in your garage!
Today for me was working on the rear wheel of my 650. One of the few things I got out of the mechanic up the road was a rim, power coated rear hub and a spoke kit. I made my self a nice wheel out of those parts, and then began looking around for a drum and brakes.
Last year I bought a serviceable whole rear wheel including brakes for $80. I'm running it on the 500. This year there's been only one I've seen (on eBay) and it went for over $425! Used!!
So I dragged out the rusted-solid rear that came with the 500 and tried once again to get the brakes out of the drum. Couldn't budge it or pry it, let alone turn it. So in desperation, I drilled out the pivot post which allowed the shoes to finally pop free of the drum.
I think I can have the drum turned and still have enough thickness left to use as a rear brake - I'll take it to my brake-guy in town, but I may be living in fantasy land.
If any of you all's have some spare rear parts please shoot me a PM.