I converted my PV before I put it back on the road in the early 2000's. At the time I was able to scrounge up all the parts required. the caliper brackets came off a 122 as did the rotors. Two years ago I replaced the rotors with ones from VP for around $120 each. I installed a proportioning valve from "Stainless Steel Brakes", in reality it is a biasing valve to control fluid flow to the rear wheels. It has a adjustable valve that with a couple of hard stops allowed me to set the biasing to stop rear wheel lock up. I just need to keep the rear drums adjusted once a year and no trouble since.
I tried the kevlar shoes and pads from Porterfield, I can't say as I notice any appreciable difference

Note
I am currently using 240 nonvented calipers ( I got these from Rock Auto) although 120 ones work too. The 3 piston calipers are now available new from VP along with the rotors and rear drums for $120 each. the flex hoses are available there too.

I rebuilt the 3 piston caliper on my son 123GT and found a supplier in New York state who sells the kits with stainless steel pistons. VP also has these but were out of stock last year when I needed them.

Steve in Canada