Hello and Welcome to the Forum.
Do you have a picture? How did the stain get there? On the glass or under the glass? Is it rust? Rainwater grit? I would try rinsing out the speedo with rubbing alcohol and compressed air. Test a small spot first to make sure it does not remove the ink on the numbers. Get the 90% or better solution cause the 70% can leave a residue.
I had lots of water in the speedo on my Sportster. I'm not sure if it was rain or condensation from the air on rainy days (it was almost always garaged at night). The clean water would not go away without help. I used a magnifying glass to dry out the condensation in the speedo. I purposely found a sunny spot outside, but it did not dry fast enough for me. Just be careful not to concentrate the heat in one place for too long-- one of the numbers began to "bubble". I never had a stain, it didn't sit like that for long.
Good Luck!
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