I've seen all sorts of gasket types used, from cut down copper/asbestos to glass wool. So long as the thread is still good, and you tie-wire the nut, there are lots of make-do options, if only to get you by until Jared sends you the right one. I haven't had a look lately, but I might have a used one floating around for a 54m in my pile of Montesa bits. I'll have a look tomorrow.
Mike