From the numbers you listed, I'm guessing those are metric speedos? And I'm guessing that the one you've been offered is a taller ratio. Perhaps it's the one that came with the R80ST for the optional taller final drive. If you install that one, you'll probably read close to right on or maybe lower than actual speed, but the odometer will show less than actual distance travelled.
The bezel will need to be removed to replace the speedo, so why not take a look at it first? A lot of times the problem is water entering through the reset button and rusting the mechanism. Perhaps the rust can be cleaned up, the mechanism lubed, and it'll work again.
Worth a try!