Depends on what's wrong with them most of the time the gears turn into cheese and disintegrate apparently caused by the lubricant.Otto instruments 124 Evans Rd, Salisbury QLD 4107 (07) 3277 3888 will replace them,or you can buy your own gears, look on internet there are four different sets metric & imperial two types of speedo.You need to count number of teeth.Alternatively try ebay for a new cluster sometimes they come up occasionally at a reasonable price.
Regards Martin