Can't help with a replacement gearbox I'm afraid but I can say (from experience) that they were well known for linkage problems so maybe that's a good (and hopefully inexpensive) place to start?
I replaced the linkage in my Volumex with some rose-jointed masterpiece that an enthusiastic genius in the US had manufactured. I guess this might be of limited use now that the gearbox is not in a Beta but I notice that BetaBoyz have some useful bits here.
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