OK the wheel thing as a brain fart on my part, I was trying to visualise the thing instead of just getting up and looking at it
The brake band runs over the end of the bobbin, not a separate wheel, sorry.
To DIY the brake band assembly you'll need 2 eyelets or cup-hooks, some fishing line or nylon thread, 2 small springs - the kind with a ring at each end, though you could McGyver those with a pair of pliers - and a short piece of dowel.
There's an eyelet screwed in to the wood here, holding a small spring to which the brake band is tied:
The brake band runs over the end of the bobbin. To remove the bobbin you just lift the band off with a finger.
There's another eyelet at the other side - the band runs through that and is then tied to one end of a spring:
Another section of line is tied to the other end of the spring, and to the tension knob. You could just use a piece of dowel that fits in the hole, the brake band is wrapped round it and you turn it to adjust the tension.
Hope that helps!