How did you get dirt up there? Is one of the lights or plugs in the bottom of the speedometer missing?
I can't think of any way to clean it - except for sending it to North Hollywood Speedometer. They can take the sealed ring off, clean the glass, and then put it back together.
http://www.nhspeedometer.com/It most likely will cost more than getting a good used speedometer with a clean face. Just watch eBay for one with similar mileage that is in good shape. Everybody building a RYCA is ditching their stock speedo.