Alright, I got my brake adapter. It looks to be pretty solid. The first pic is the adapter with its hose clamps and the brakes that I salvaged off of an old bike. The second pic shows where it will be mounted. I believe it's supposed to be mounted on the upper part of the rear fork and the top hole gets a bolt through it to that little cross bar. I'm not going to do it that way. For one the hole in my cross bar is at the wrong angle for that to work, for another it doesn't leave any room for a fender up there. My plan it to weld a steel L-bracket to the brake adapter so that the bracket goes downward. Then bolt the bracket to a threaded hole that's down there already. I'll also need longer hose clamps. All of that will have to wait until I get an engine because positioning of the adapter will depend on where the wheel sits with the engine chain on and tensioned. Also, I don't plan on using a chain tensioner on the engine side either.
It's going to be a while before it's all operational. I'm building on a shoestring budget, but I'm unwilling to compromise on anything. That just means that it takes longer to save up and get things.