Instrument Cluster Replacement

Hi Guys & Gals

Quickie question.
The speedometer & mileometer on my 91 clipper is faulty. the speedo never goes above 40 mph & the mileometer rotates at the speed the speedo is showing, so if i say do a 20 mile trip the mileometer will say i've done 10 (give or take)
my mechanic says it most likely isnt the speedo cable and more likely the instrument cluster.
So if i get a replacement cluster can i replace it with anyone from any mk1 golf / gti / cabby etc, or does it need to be a specific one?

Many thanks

any mk1 golf cluster will do so long as its the same make (vdo or motormeter) plus it has to be from a big block mk1 1.5-1.8 as the small block speedo is calibrated differently

ok great thanks

can i just ask before i start spending more money on parts.
is there anyway of determining what part would be broken ie: speedo cable or instrument cluster.
dont really want to buy another cluster if its the cable

definitely the speedo, the cable wither spins or it doesnt. plus it wouldnt be able to make the milometer spin at a different speed than the speedo