How "old school" do you want to go? Drum brake with twin leading shoes? I have a Yamaha R3 (about 1969) TLS front drum brake set up. It uses a 17mm axle and can be used on XS650 forks.
However, it is a lot of work. You'll need to make wheel spacers, a brake stay, and modify the brake panel with a brake stay bolt, and if you want a speedometer, find a early speedometer cable the right length for your bike. As for a rim, you might need to send the hub to a place like Buchanan's and have spokes made and a 21" rim drilled to suit.
Just for an example, I made up a "tribute" XS1 wheel out of one of these brake assemblies by affixing fake cooling hole plugs and lacing it to a 19" rim. Photos below show it on an SR500 (same forks as XS650).
If you want to see photos of the actual brake assembly, let me know and I'll shoot some and post them.
Bill