Ive debated scanning a speedo face, photoshopping as needed then reprinting onto an 8.5x11 sheet of 'sticker paper' - then sticking that over the entire face and seeing how the lights shine through.
sticker paper is available at any officemax/staples, but its longevity, glare and print quality may not be a step up from where you are now.
This also would not represent black well (ie - you'd do better to have black numbers on a white background
you could add icons, symbols, fix funny characters or otherwise personalize the faces.
you would need to cut some very nice circles in key places.... not sure how good this would look if it was done at home.

but if you had a spare/broken cluster..it might be worth trying.

most clusters Ive seen have a speedo needle held on by friction - you use care and multiple small levers to pull it off. put some vinyl electrical tape on the existing face to have a 'durable' surface to pry against.

voltage vs speed - could you pull the cluster, drive the car at a known speed and measure voltage generated by the speedo sensor ? usually that's a pulse so just measuring it with a Voltmeter might not work well.
or better get - get a spare speed pickup, wire it up with +12 to the cluster. Put the sensor next to your drill and try to simulate a set speed (ex: full drill speed = 85mph) then when you re-assemble, you can check it this way.
none of these are easy...how does the factory check the calibration on a speedometer ?

-John

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2009 Kia Sedona
2009 Honda Odyssey EX-L
12006 Jetta Pumpe Duse
(insert Mercedes here)

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