There aren't any "new" Smiths gauges available anymore. I just got mine back from Nisonger in NY. Had them do a total refurbish of my speedo. Looks brand new. About $130.
You could take it apart yourself and try to fix it but chances of having availability of gears, magnets, etc. is nil. Nisonger has two services available, slow at regular price (4-6 weeks) or really fast (1-2 days) for an additional $35. Your choice.
Bill