First, that's THE maddest head angle I think I've ever seen, besides a bike which has been involved in a front end impact!! I know the Mantras always had steep head angles, but your pictures do a good job of showing just how steep they are!
Second, I used to have a No.2 caliper on the back of an old LTS of mine, and I was running a Hope Bulb hub with it, which is basically the slimmed down version to what you have (which is a Big 'Un), and the predecessor to all of the later incarnations of Hope Hubs and, in my opinion, the best hubs that they ever produced
Your other option would be to find a Ti Glide rear hub with a similar setup to your front hub.
I can't remember how the caliper mounts to the axle so I can't tell you whether or not you'd be able to run any other hubs with it.
Bear in mind that if you swapped the rear wheel/hub, then you might have to get a new disc rotor too unless you can find a 5 bolt Hope Bulb or Hope Ti Glide. Most disc hubs are 6 bolt.