Some time back I rang a company from MCN classifieds that advertised MPH speedo exchanges and asked them about this. Answer - they carefully cut round the casing of the speedo & replace the neccessary then glue it back together.
The join is hidden & it should be fine if done with care I would have thought. Any way it sup to U, good luck.
They supply complete replacement dial faces with installation instructions so they should know how to take a speedo apart the right way (and get it back together again!).