Nick H,
Without seeing a picture of the broken wire, I don't know how best to advise you. Is the broken wire on the armature or the round outer steel shell with the field coils? If its either of these then I would recommend taking it to a local garage with a mechanic who can look at it and see it's repairable. Each brush has a woven copper wire attached to it, if its one of them that has the broken wire, then get a new set of brushes and put it back together, stick it back your cubbie and repolorize it and you should be good to go. If replacement of the S/G turns out to be the appropriate course of action, you might check out ebay as you can get a good used one there for around $30-40 or send me an email and we will see if we can help you out.