It would pay to check the drive before blaming the speedo.
So with your dash out and the rear wheels up on stands put the car in gear and see if the cable actually spins. If it doesnt you will have to trace back to see if its a busted cable, a problem in the 90deg drive or in the drive gear in the rear of the gearbox.
One thing i noticed when doing mine is that the drive gear in the gearbox is metal, the Gemini ones have a blue plastic gear.
The on on the left is out of my Piazza. The one on the right is heavily worn and out of a Gemini.
To change it you undo the 12mm headed bolt and work it out of the gearbox.
Take care no to snap the tab off the bolt goes thru.
Never use the tab to lever with a screw driver, use you hand to work it out the first bit so you can get a screwdriver in under the main part.
Watch out when you pull it out it will dump out a fair bit of oil.
Make sure the new drive has the same amount of teeth as the old one.
Dont use the bolt to pull the new drive in to the box you will break the tab.
Tap it in with the plastic handle of a screw driver.
Cheers. Bob.