Motorcycle Repair Questions? Ask a Mechanic ASAP
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I'm not entirely familiar with bikes, but have you by any chance checked the speedo cable? it should run off the back of the speedometer to the front wheel. You may have to remove some panels to access the cable.
Have a look to see if the cable appears damaged or frayed.
Given the age of the bike it might be worth changing this cable first as it is a cheap and easy fix. They are roughly £5 or less online.